A woman places a candle at the site of Tuesday's suicide bomb attack at Sultanahmet square in Istanbul, Turkey January 13, 2016. REUTERS/Osman Orsal

By Ayla Jean Yackley and Humeyra Pamuk

ISTANBUL (Reuters) - An Islamic State suicide bomber who killed 10 German tourists in the heart of Istanbul's historic district entered Turkey as a refugee from Syria and went undetected as he was not on any watch lists, Prime Minister Ahmet Davutoglu said on Wednesday.

The bomber, who blew himself up among groups of tourists on Tuesday near the Blue Mosque and Hagia Sophia, the top sites in one of the world's most visited cities, had registered with immigration authorities in the city a week ago.

Turkey has kept an open border to refugees from Syria's civil war and is now home to more than 2.2 million, the world's largest refugee population. But its border has also been used by foreign fighters seeking to join Islamic State or return from its ranks to commit atrocities abroad.

"This individual was not somebody under surveillance. He entered Turkey normally, as a refugee, as someone looking for shelter," Davutoglu told a news conference, adding he had been identified from fragments of his skull, face and nails.

"After the attack his connections were unveiled. Among these links, apart from Daesh, we have the suspicion that there could be certain powers using Daesh," he said, using an Arabic name for Islamic State.

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Syrians living in Istanbul hold candles and flowers at the site of Tuesday's suicide bomb attack …

Turkey accuses Syrian President Bashar al-Assad, and his allies including Iran and Russia, of cooperating with Islamic State in the Syrian regime's effort to destroy Syrian opposition forces.

Turkey, which like Germany is a member of the U.S.-led coalition against Islamic State in Syria and Iraq, has become a target for the radical Sunni militants.

It was hit by two major bombings last year blamed on the group, in the town of Suruc near the Syrian border and in the capital Ankara, the latter killing more than 100 people in the worst attack of its kind on Turkish soil.

Asked if Turkey planned retaliatory air strikes on Islamic State, Davutoglu said Ankara would act at a time and in a manner that it saw fit. He pointed out the Turkish military had hit Islamic State targets abroad after the Suruc and Ankara attacks.

But he said Russia's entry into the Syrian war was a complicating factor. Turkish war planes have not flown in Syrian air space since Turkey shot down a Russian fighter jet in late November, triggering a diplomatic row with Moscow.

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Syrians living in Istanbul hold candles at the site of Tuesday's suicide bomb attack at Sultanah …

"They (the Russian air force) shouldn’t obstruct Turkey's fight against Daesh ... Right now unfortunately there is such a barrier," Davutoglu said. "Certain countries are in an obstructive attitude in terms of Turkey’s air bombardments. They should either destroy Daesh themselves or allow us to do it."

TOUR GUIDE YELLED "RUN"

Asked about a report in the Turkish media that the bomber had registered at an immigration office in Istanbul a week ago, Interior Minister Efkan Ala earlier confirmed that his fingerprints were on record with the authorities.

The Haberturk newspaper published what it said was a CCTV image of the man, named in some local media as Saudi-born Nabil Fadli, at an Istanbul immigration office on Jan. 5. Turkish officials have said he was born in 1988.

Foreign tourists and Turks paid their respects at the site of the attack early on Wednesday. Scarves with the Bayern Munich soccer club emblem were left along with carnations and roses at the scene, before Turkish police sealed off the area.

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A person pays his respects at the Obelisk of Theodosius, the scene of the suicide bomb attack, at Su …

German Interior Minister Thomas de Maiziere, visiting Istanbul, said there were no indications Germans had been deliberately targeted and that he saw no reason for people to change travel plans to Turkey. He said Germany stood resolutely by Turkey's side in the fight against terrorism.

"If the terrorists aimed to disturb, destroy or jeopardize cooperation between partners, they achieved the opposite. Germany and Turkey are becoming even closer," he said, adding there was no link to Germany's role in the fight on terrorism.

Davutoglu praised the German group's Turkish guide who, according to the Hurriyet newspaper, yelled "run" after seeing the bomber standing among the tourists and pulling a pin on his explosives, enabling some of them to get away.

Witnesses said the square was not packed at the time of the explosion, but that several groups of tourists were there.

"I didn't finish the tour, you know, the tour I had bought," said Jostein Nielsen, a wounded Norwegian tourist, as he waited on a stretcher at Istanbul airport, his left leg bandaged.

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Turkish Prime Minister Ahmet Davutoglu (C) and his wife Sare lay red carnations in front of the Obel …

"I still have to go to the Blue Mosque and the old Turkish Bazaar ... We have no hard feelings towards Turkey. We know there are some mad people out there," he said.

DETENTIONS CONTINUE

Davutoglu said the security forces had detained four people suspected of links to the suicide bomber, and that six of those wounded were still in hospital. The German foreign ministry said earlier five Germans were still in intensive care.

A Peruvian national was also injured in the blast.

Turkey has rounded up hundreds of suspected Islamic State members since launching what it called a "synchronized war on terror" last July, raids which continued on Wednesday.

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Turkish Prime Minister Ahmet Davutoglu and his wife Sare (L) pay their respects in front of the Obel …

Since the attack, police have detained a total of 65 people including 16 foreign nationals in six Turkish cities, the Dogan news agency reported.

The Russian foreign ministry confirmed three of those detained were Russian nationals, but it was not immediately clear whether there was any connection to the Istanbul attack, for which there has been no claim of responsibility.

Turkey has faced criticism at home and abroad for failing to do more to fight Islamic State networks, but Ala, the interior minister, defended Turkey's record, saying 200 suspects had been detained just a week before the Istanbul blast.

He said Turkey, which has repeatedly called on foreign intelligence agencies to do more to prevent would-be jihadists from traveling to its shores, had detained 3,318 people for suspected links to Islamic State and other radical groups since Syria's conflict began. Of that number, 847 were subsequently arrested, most of them foreigners.

(Additional reporting by Melih Aslan in Istanbul, Orhan Coskun in Ankara, Michelle Martin in Berlin, Jack Stubbs in Moscow; Writing by Nick Tattersall; Editing by Giles Elgood and Janet McBride)

The gains have been limited, and have relied heavily on support from mostly Shiite foreign fighters from Lebanon's Hezbollah movement, as well as Afghan and Iraqi forces, and Iranian "advisers".

But they have allowed the regime to retake the initiative and go on the offensive after a humiliating string of defeats.

Perhaps the regime's biggest success since Russia began air strikes last September was this week's capture of Salma, a town in coastal Latakia province that became a rebel stronghold after its 2012 capture.

Simultaneously, the army is seeking to encircle the city of Aleppo, advance in the south of central Hama province and east in Homs, and is on the offensive in the key rebel town of Sheikh Miskeen in southern Daraa province.

"The Russian intervention has undoubtedly been of immense value to the Syrian regime, which was very much on the back foot in mid-2015," said Torbjorn Soltvedt, head of Middle East and North Africa at the Verisk Maplecroft risk analysis company.

"The intervention has largely succeeded in halting rebel advances led by the FSA (moderate Free Syrian Army rebels) and the Islamist Army of Conquest, and eased mounting pressure on the regime," he said.

The Army of Conquest rebel alliance, which includes Al-Qaeda affiliate Al-Nusra Front, dealt the regime a particularly severe blow last year with the capture of all of Idlib province.

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Russian servicemen assist air force pilots in a Sukhoi Su-30SM fighter jet before departure on a mis …

- Can regime hold territory? -

But Russia's strikes have brought new pressure to bear on opposition forces, with the Britain-based Syrian Observatory for Human Rights saying Moscow carried out some 120 strikes on Salma before its capture by Syrian regime forces.

Soltvedt said Russia's air campaign would "continue to play a hugely important role in the conflict," but he cautioned "there is little to suggest that the Syrian army can roll back gains made by the rebels in 2015 in the short term."

"The regime is certainly on the offensive," said Firas Abi Ali, a senior analyst at research firm IHS Janes.

"Its weapons superiority and Russian aerial support mean that the far-more-poorly-equipped insurgents will have to yield territory," he told AFP.

"The question is whether the Syrian army has the capability to hold the territory that it takes. Its manpower shortages, and insurgents' access to friendly borders, have limited its ability to do that in the past."

Experts also noted that the regime's gains so far have not been geographically vast.

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Syrian rebels seized control of the town of Salma in the coastal province of Latakia in 2012 (AFP Ph …

French geographer and Syria expert Fabrice Balanche said the advance in Salma, along with the recapture of the Kweyris military airport in Aleppo and additional wins in southern Aleppo amounted to about 400 square kilometres (155 square miles) of retaken ground.

- New army morale -

"The army has regained morale, that's for sure," said Balanche, a visiting fellow at the Washington Institute for Near East Policy think-tank.

But he too noted the regime was relying heavily on not just Russian support but also Shiite militants.

"Russia's goal is to clear the Jabal Turkman and Jabal Akrad regions (of Latakia) because rebels there can directly threaten Russian bases in the province," he said.

"Then comes the recapture of Aleppo, because Assad wants to remain president of Syria and Syria means Aleppo and Damascus. If he rules only Damascus, he's only a half-president."

Vasily Kashin, an expert at Moscow's Centre for Analysis of Strategies and Technologies, said Russia has been able to "partially" achieve its main goals: preventing the regime's collapse and creating more favourable conditions for talks.

"The situation has stablised, the government is no longer threatened with collapse, Syrian troops are advancing, albeit slowly, and negotiations have started, with... participants no longer insisting on Assad's immediate departure," he said.

In the south meanwhile, a regime push for the town of Sheikh Miskeen could be followed by a bid to recapture the nearby Daraa and Nasib border crossings with Jordan, which were captured by rebels in October 2013 and April 2015 respectively.

Jordanian political analyst Labib Qamhaoui said Amman could favour a regime recapture of the border posts, despite its support for the opposition, to "reduce the security burden the kingdom currently has in protecting both its border and Syria's."

Whether or not the army is able to retake the crossings, analysts said the advances did not ensure the regime, which controls around 30 percent of Syria's inhabited areas, would be victorious in the long-term.

"Ultimately, the Russians need to help the Syrians cut off insurgent supply lines and allow Syrian government forces to re-establish themselves along Syria's border," said Abi Ali.

"This requires the cooperation of the Jordanian and Turkish government, however, which is impossible to secure outside the context of a political settlement."

And last July, researchers reported that global warming is working to “crush bumblebees in a kind of climate vice,” according to Nature.

“Bumblebee species across Europe and North America are declining at continental scales,” Jeremy Kerr, a biodiversity researcher at the University of Ottawa in Canada, told the scientific journal. “Our data suggest that climate change plays a leading, or perhaps the leading, role in this trend.”

It’s not only animals that are at risk. Researchers believe that the western drought killed 12 million trees in California’s forests, and estimated 58 million are so dry they’ve reached the brink of death, according to the San Francisco Chronicle. A study released that last month predicted that climate change would cause massive die-offs of the American southwest’s coniferous trees, like junipers and pinon pines, within the next half century.

Not every die-off of the past year can be blamed on climate change. Two “unusual mortality events” involving endangered Guadelupe fur seals — which were being stranded at eight times the normal rate on California’s central coast — and large whales in Alaska — where scientists have found the decomposing carcasses of more than 30 unlucky animals — have been loosely linked to that weird warm “blob” out in the Pacific. And the causes of other incidents — the recent whale stranding in India, for example — remain undetermined. Typically mass strandings are linked to toxic algae blooms, disease and trauma, and changes to the animals’ habitat, marine mammal expert Darlene Ketten told Scientific American in 2009.

But a study published in the Proceedings of the National Academy of Sciences last year suggests that the number of animal die-offs has gotten worse in recent years. And the researchers weren’t talking about small scale problems like the murres deaths or even the saiga die-offs either. They looked at more than 700 mass mortality events in which either 90 percent of the species was wiped out, more than a billion individuals were killed or 700 million tons (nearly 2,000 Empire State Buildings) worth of biomatter was destroyed.

What they found was not heartening. Mass Mortality Events (MMEs) are “rarely placed in a broader context,” the study’s authors reported. But they seem to be happening at an increased rate for birds, marine invertebrates and fish since the 1940s — even when researchers took into account that such events are more likely to be reported now than they were 75 years ago.

These die-offs matter not just because of the inherent value of the creatures involved, the authors said, but because whole ecosystems may depend on that species to survive.