Some 10,000 ISIL militants held in prisons in northeastern Syria present a major security risk, a senior State Department official said on Tuesday, urging countries to take back their citizens who joined the group and were then detained.

"It's a ticking time bomb to simply have the better part of 10,000 detainees (held in Syria), many of them foreign fighters," the official told reporters in a conference call.

ISIL has lost almost all of its territory in Iraq and Syria. Its former leader Abu Bakr al-Baghdadi was killed in a U.S. raid last month but the group remains security threat in Syria and beyond.

Allies have been worried that ISIL militants could escape as a result of Turkey's assault against Syrian Kurdish militia fighters, who have been holding thousands of the group's fighters and tens of thousands of their family members.

The U.S. official said little progress had been made on the repatriation of ISIL detainees, with only some taken back by certain Middle Eastern and Central Asian countries.

"Given that there are hundreds of people being held from Europe, we are very troubled by this," the official said. "It's a major issue of diplomatic discussion," he added.

The United States will hold a meeting of foreign ministers from the U.S.-led coalition fighting ISIL in Washington on Thursday to discuss the next step towards recalibrating the fight against the jihadi hardline group.

The issue of how to handle ISIL detainees is likely to take the center stage.

Trump cleared the way for a long-threatened Turkish incursion into northeastern Syria on October 9 against Kurdish forces who had been America's top allies in the battle against ISIL since 2014.

The official said the United States was confident that in the meantime, Syrian Kurdish militia can keep the detainees secure but does not want to take any risks by having such a large group of militants in one place.