WASHINGTON (REUTERS) - Some 10,000 Islamic State in Iraq and Syria (ISIS) detainees held in prisons in northeastern Syria present a major security risk, a senior State Department official said on Tuesday (Nov 12), urging countries to take back their citizens who joined the group and were detained.

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

ISIS has lost almost all of its territory in Iraq and Syria. Its former leader Abu Bakr al-Baghdadi was killed in a US raid last month but it remains security threat in Syria and beyond.

Allies have been worried that ISIS 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 official said little progress was made on the repatriation of ISIS detainees, with only some taken back by some Middle Eastern and Central Asian countries.

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

The United States will hold a meeting of foreign ministers from the US.-led coalition fighting ISIS in Washington on Thursday to discuss the next step on how to recalibrate the fight against the Islamic hardline group.

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

Trump cleared the way for a long-threatened Turkish incursion into northeastern Syria on Oct 9 against Kurdish forces who had been America's top allies in the battle against ISIS 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 a such a large group of militants in one place.