The land and air offensive launched by Turkey against a Kurdish-led force which spearheaded the battle against Islamic State in Syria has alarmed Western allies, who fear it could result in dangerous militants escaping to target the West.
The Kurdish-led Syrian Democratic Forces (SDF) are holding thousands of Islamic State prisoners across the region and have said they cannot guarantee their continued detention while they confront Turkey's incursion.
The exact locations of the detention centers has not been disclosed but a Turkish security official said Ankara believed there were several in the 30-km (20 mile) deep border area where it says it will establish a "safe zone" under its control.
"It is estimated that there are approximately 1,200-1,500 Islamic State members in buildings being used as prisons inside the region where Turkey is carrying out its ... operation," one official, speaking on condition of anonymity, told Reuters.
The official said Turkey had not received official notification, but cited information "from various sources in the field" saying they included large numbers of foreign fighters from the United States, France, Germany and Belgium.
"The foreign terrorist fighters will not be released," the official said. "However, there will also be initiatives to send them back to their countries of origin."
He did not say how Turkey could ensure that the detainees remain in captivity in the event of fighting or shelling around the prisons.
The Syrian Kurdish-led authorities said on Thursday that Turkish forces had shelled a jail holding Islamic State prisoners of more than 60 nationalities in the city of Qamishli, calling it a "clear attempt" to help them escape.
The border city lies well inside Turkey's proposed "safe zone," but it is also in the far east of the country. For now, Turkey's land incursion appears focused on the two towns of Ras al Ain and Tel Abyad, much further west.
2,000 FOREIGN FIGHTERS
A U.S. official told Reuters there are currently 30 detention centers holding Islamic State prisoners across northeast Syria. They hold 11,000 Islamic State detainees, of which 2,000 are foreign fighters, the official said.
"For now the ... prisons remain and they are being guarded by the SDF. There has been no change there," the official said.
Earlier this week the United States said it had taken custody of two high profile Islamic State militants. An official identified them as members of a group of British fighters tied to the killings of Western hostages.
Turkish President Tayyip Erdogan's communications director said that U.S. President Donald Trump, in a Sunday phone call which appeared to pave the way for Turkey's offensive, agreed "to transfer the leadership of the counter-Islamic State campaign to Turkey."
Ankara insists, despite international fears to the contrary, that there will be no let-up in the campaign.
"Every aspect of Daesh (Islamic State) - countering Daesh terrorism, dealing with prisoners, and women and children - Turkey will do it best like it has before and will do it in Syria too," Foreign Minister Mevlut Cavusoglu said.
Cavusoglu said it was not clear how many Islamic State militants were in the zone targeted by Turkey's incursion, estimating around 1,000 were held in prisons "and they say there are another thousand in camps," he said without elaborating.
"If those camps or prisons are in the safe zone, we are responsible. I'm not responsible for Daesh camps in the south of Syria," he told a group of journalists including Reuters in Ankara on Thursday.
"The U.S. and others made it clear they will be staying in the southern part...," he said. "So it will under the Americans' responsibility."