Around 2,000 foreign Islamic State (IS) fighters are believed to be detained by US-backed forces in Syria, CNN reported.
A thousand foreign fighters have so far been confirmed in a screening process that involves biometric assessment and the number is expected to double.
"Currently, we can confirm more than 1,000 foreign terrorist fighters from more than 50 countries in SDF custody. We anticipate that number will rise as we work with the SDF to verify the national identities of ISIS fighters in SDF custody," Pentagon spokesman Cdr. Sean Robertson told CNN.
Up to 9,000 IS fighters are being held by the Kurdish-led Syrian Democratic Forces (SDF), while 60,000 of their "associates" - men, women and children linked to IS - are staying in makeshift camps.
"These are women and children, and the vast majority of these are assessed not to be sort of innocent civilians these are people who have chosen to stay, or were coerced to stay as part of the ISIS caliphate that remained," a senior US defense official told CNN last month.
The officials shared with CNN that "a very small number" of the foreign fighters were US citizens. One US official put the number at "less than two dozen".
In the most high-profile US case last month said that Hoda Muthana, a young woman from Alabama who joined the Islamic State group, was not a citizen due to her father's former diplomatic status.
Many women and children like Hoda are living in the al-Hawl camp in Syria which has a current population of 73,041 people, 65 percent of whom are children under the age of 18, according to the UN.
The camp's population - which has grown by more than 60,000 since December, is made up of 15 percent foreign nationals.
The SDF has warned it is struggling with the intake of vast amounts of people from former IS-held areas and has called on countries to repatriate their citizens to ease the burden on Syria.
Repatriation is a sensitive issue for Western nations such as France and the UK, which have experienced attacks by homegrown extremists and have little interest in seeing more return.
The UK has gone so far as to strip Islamic State group proponents of citizenship as it did with the case of Shamima Begum.
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