By Mirwais Harooni and Kay Johnson

KABUL (Reuters) - A suicide car bomber killed two U.S. troops and a Polish soldier in an attack on a convoy near the U.S. embassy in Kabul on Tuesday, the NATO-led coalition said, one of the worst attacks on international forces in the Afghan capital in months.

The attack near the heavily fortified embassy wounded 13 Afghan civilians nearby. It came amid a months-long political stalemate and an emboldened insurgency, with a presidential election still unresolved as most foreign combat troops prepare to leave by the end of the year.

The Taliban claimed responsibility for the bombing on the main road leading to Kabul's international airport, not far from the sprawling U.S. embassy compound that is also home to other members of the NATO-led International Security Assistance Force (ISAF) coalition.

In a text message to journalists, a Taliban spokesman said the suicide bomber, identified only as Bilal, had been lying in wait for foreign troops in a car packed with explosives.

Two of the three killed were American military personnel, a U.S. Defence Dept. official confirmed on Tuesday in Washington.

Poland's military identified the third victim as Rafal Celebudzki, a platoon commander who had also served in Iraq. Two other Polish soldiers were wounded.

Afghan Deputy Interior Minister Mohammad Ayub Salangi said on Twitter a suicide car bomber had targeted a convoy of vehicles carrying foreign troops.

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U.S. troops carry the dead body of a soldier from a NATO-led international military force, at the si …

The blast tore through cars and shattered shop windows on the road a few hundred metres from the main embassy gate.

Kabul police spokesman Hashmat Stanekzai said 13 Afghan civilians were wounded and 17 nearby cars damaged.

"HUGE BLAST"

Ambulances arrived within minutes of the blast during the morning rush hour at about 8 a.m. (0330 GMT).

"It was a huge blast," said wounded bystander Haji Awal Gul, his shirt splattered with blood as he stood on the roadside.

Tensions have been building in Afghanistan since the disputed run-off vote in a presidential election in June, with rival candidates still arguing over the outcome despite U.S. efforts to broker a compromise deal.

Taliban insurgents have been exploiting the uncertainty, launching bombings and attacks on government security forces and officials across Afghanistan.

In western Herat province, one U.S. soldier was killed when an unidentified member of the Afghan security forces turned on his trainers late on Monday, the latest incident in a string of "green-on-blue" attacks.

A Western official, who asked not to be identified, said the U.S. soldier was killed when the Afghan threw a hand grenade at his trainers.

ISAF did not confirm the exact details of the attack. An ISAF statement on Tuesday said the U.S. service member was killed when an individual wearing an Afghan army uniform "turned his weapon against ISAF members".

Also overnight, two suicide bombers set ablaze 26 fuel tankers in an attack on a customs post in eastern Afghanistan near a border crossing into Pakistan, Afghanistan's TOLO media reported on Tuesday.

One of the attackers detonated explosives he was carrying while the other was killed in an ensuing gunbattle, the report on TOLOnews.com quoted Afghan officials as saying.

(Additional reporting by Missy Ryan in Washington and Jessica Donati in Kabul; Editing by Paul Tait and Clarence Fernandez)

By Stephanie Nebehay

GENEVA (Reuters) - United Nations war crimes investigators for Syria cautioned world powers preparing military action against Islamic State fighters that the 'laws of war' would apply to them and they must do everything possible to protect civilian populations.

They said ISIS, which controls swathes of Iraq and Syria, had committed massacres of religious and ethnic groups and abused women. The government of President Bashar al-Assad, however, remained responsible for the majority of civilian casualties.

World powers meeting in Paris on Monday gave public backing to military action to fight Islamic State fighters in Iraq. The U.S. military struck an Islamic State target southwest of Baghdad, U.S. Central Command said, in an expansion of the Obama administration's campaign against the group that has seized large swathes of Iraq and Syria.

"As military action on ISIS positions seems increasingly likely, we remind all parties that they must abide by the laws of war and most particularly, the principles of distinction and proportionality. Serious efforts must be made by all parties to preserve civilian life," Paulo Pinheiro, chairman of the U.N. commission of inquiry, told the Human Rights Council on Tuesday.

Air strikes against Islamic State fighters have already stirred complaints in Sunni areas of civilian casualties. Prime Minister Haider al-Abadi said on Saturday he had ordered his forces to refrain from any strikes in civilian areas, even towns occupied by IS militants.

The rules of war embodied in the Geneva Conventions require warring sides to distinguish between military and civil objects, such s schools and hospitals, and to carry out operations that are proportionate to the perceived threat.

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In this photo released by the Syrian official news agency SANA, United Nations special envoy to Syri …

Washington has been trying to build a coalition to fight Islamic State since last week when President Barack Obama pledged to destroy the militant group. France on Monday sent jets on a reconnaissance mission to Iraq, a step towards becoming the first ally to join the U.S.-led air campaign there.

The countries that did attend the Paris talks made no mention at all of Syria, where U.S. diplomats face a far tougher task building an alliance for action.

Islamic State forces in Syria have brutally and publicly executed civilians, as well as captured rebel fighters and government soldiers over the past two months, Pinheiro said.

"Recent events in Iraq lay bare the threat ISIS poses for religious and ethnic minorities inside Syria," Pinheiro said.

The group killed hundreds of civilians at Al-Shaar gas fields in eastern Homs and there are reports of hundreds from the al-Sheitaat clan in Deir al-Zor being executed, he said.

"Women living in ISIS-controlled areas have been banned from public life...Some women have been stoned to death, ostensibly for adultery," Pinheiro told the Geneva forum.

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People carry a remnant of a war plane that crashed on the outskirts of Raqqa in northeast Syria Sept …

Indoctrination of children is a priority for the insurgents and youth are made to take part in hostilities, he said.

The group is "the most recent beneficiary" of inaction by the U.N. Security Council and the prevailing impunity, according to the independent experts who include former U.N. war crimes prosecutor Carla del Ponte.

"STARVATION OR SUBMISSION STRATEGY"

But the government of President Bashar al-Assad "remains responsible for the majority of the civilian casualties, killing and maiming scores of civilians daily - both from a distance using shelling and aerial bombardment and up close, at its checkpoints and in its interrogation rooms," Pinheiro said.

"In the last two months, Deraa has been heavily bombarded with little sign of the government's attempting to distinguish between civilians and members of armed groups," he said.

Many areas under bombardment or siege are yielding to local truces, a "measure of the success of the government's 'starvation or submission strategy'," he said.

"Checkpoints are often the starting point of a horrific journey of disappearance, torture, sexual abuse and, for many, death," Pinheiro added.

The U.N. investigators released 12 testimonies from among 3,200 collected in their interviews with Syrian refugees and defectors in neighboring countries and by Skype within Syria.

"They include a child injured in a missile attack on a school in Aleppo city, a man tortured in Damascus Mezzeh detention facility, and a pregnant woman left adrift after losing her husband and parents," Pinheiro said.

"Thousands of men, women and children have been beaten, electrocuted and hung for hours by their wrists from walls."

Women have given "shattering testimonies" of torture, rape and sexual assault by government forces, he said.

(Reporting by Stephanie Nebehay; editing by Ralph Boulton)

U.N.: 'Laws of war' apply to Islamic State fighters

Syria war crimes investigators tell countries preparing to battle IS that they must also protect civilians.
'Starvation or submission'