Close to 100,000 people are locked out of Turkey near the Oncupinar border crossing after they fled escalating air strikes and a Syrian government assault on Aleppo, with humanitarian aid from Turkish relief agencies provided inside Syria.

Turkish President Tayyip Erdogan has repeatedly said he prefers the setting up of a "safe zone" perimeter inside Syria border where displaced people would be protected from attack.

"We hope that they are secure where they are, but we would also hope in the end there would be continued Turkish generosity," the deputy secretary-general of the United Nations, told Reuters when asked whether Turkey should open the border.

Eliasson said he had discussing the possible opening of the border during talks with his Turkish colleagues.

Turkey hosts some 2.6 million refugees from the five-year conflict, but is coming under growing pressure from the United States to secure the border more tightly because of the risk from militants travelling under the guise of seeking refuge, and, from Europe, to stem the onward flow of migrants.

"I understand that this is a tremendous strain on this nation and that they of course have their concerns ... that's not only for us to discuss, that's also for the European Union who are now feeling that these flows have to be reduced," Eliasson said.

Turkey has made a deal with the Europe to cut back the flow of migrants, more than two thousand of which continue to cross illegally from Turkey to Greece every day, in exchange for financial support for the refugees inside Turkey and for refreshing its accession process to the EU.

(Reporting by Dasha Afanasieva; Editing by Alison Williams)

The Islamic State group has collected millions of dollars in ransom for a group of Assyrian Christians it kidnapped in Syria a year ago, Christian officials and an opposition group said Monday, as the last of the 230 hostages were freed.

The release ended a yearlong saga for the Christians — many of them women and children — during which families had no news from their loved ones.

Younan Talia, of the Assyrian Democratic Organization, told The Associated Press that about 40 remaining captives were released early Monday and arrived in the northeastern town of Tal Tamr. He said the release came after mediation led by a top Assyrian priest in northern Syria.

The extremists captured the Assyrians, members of an ancient Christian sect, last February after overrunning several communities on the southern bank of the Khabur River in northeastern Hassakeh province.

Kidnapping for ransom is a main source of income for the extremists, who have captured scores of journalists and aid workers in the past few years, releasing some for large sums of money and killing others. In November, IS said it killed a Norwegian and a Chinese captive after demanding ransom for their release two months earlier.

Talia said IS demanded a ransom of $18 million for the Assyrian Christians. He said the figure was later lowered following negotiations. He said he did not know the final amount.

Osama Edward, director of the Stockholm-based Assyrian Human Rights Network, said 42 Christians, mostly young women and children, were released. The Britain-based Syrian Observatory for Human Rights also said 42 were released, including at least 17 women.

A Syrian Christian figure said the worldwide Assyrian community launched a campaign for the captives' release shortly after they were abducted. He said a bank account was opened in the northern Iraqi city of Irbil and donations began to flow in from around the world.

"We paid large amounts of money, millions of dollars, but not $18 million," said the man, who requested anonymity because he was not authorized to discuss the sensitive mediation. "We paid less than half the amount."

The official added that the fate of five Assyrians who went missing during the abductions was still unknown.

Observatory director Rami Abdurrahman said $25-30 million in ransom money was paid by businessmen and the Assyrian church, who asked that the terms of the deal remain secret to avoid allegations of supporting terrorism. Abdurrahman, whose group documents the war through activists on the ground, did not say how he got the information.

The money would be a shot in the arm for IS, which has been faced with a cash shortage in its so-called caliphate across parts of Syria and Iraq. The Associated Press reported earlier this month that the group is having a hard time meeting expenses, thanks to coalition airstrikes and other measures.

An Assyrian woman in Beirut, whose parents, brother and sister-in-law were among those abducted and released over the past year, told AP that the detention left her parents "psychologically scarred." She did not know how much was paid in the end, but heard that at some point the group was demanding up to $50,000 for one person.

IS attacked a cluster of villages along the Khabur River last year, sending thousands of people fleeing to safer areas and capturing the Assyrians over a period of three days. Over the next two days, the extremists picked up dozens more from 11 communities near Tal Tamr.

The Hassakeh province, which borders Turkey and Iraq, has become the latest battleground in the fight against IS in Syria. It is predominantly Kurdish but also has Arabs, Assyrians and Armenians.

On Friday, the predominantly Kurdish Syria Democratic Forces captured the IS stronghold of Shaddadeh in Hassakeh, where some of those kidnapped were once believed to have been held.

Many Syrian Christians, who make up about 10 percent of Syria's pre-war population of 23 million, left for Europe over the past 20 years, with the flight gathering speed since the country's conflict began in March 2011.

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Associated Press writer Zeina Karam contributed to this report.

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Follow Bassem Mroue on Twitter at www.twitter.com/bmroue