(CNN) The number of suspected foreign ISIS fighters being detained by US-backed forces in Syria has now surpassed 2,000, with a small number claiming to be US citizens, three US officials told CNN.

A US defense official tells CNN that the detainees are being biometrically screened and only a little over 1,000 have been confirmed to be foreign nationals but that the number is expected go up as the screening progresses.

"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.

The foreigners are among the more than 9,000 ISIS fighters being held by the US-backed Syrian Democratic Forces, with the majority of those being Syrian and Iraqi nationals.

An additional 60,000 of what the US government refers to as ISIS "affiliates" are staying in make-shift camps where the SDF has a security presence.

"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.

'Small number' claiming to be US citizens

That group includes "a very small number" of individuals claiming to be US citizens, according to two US officials.

"We are aware of a very small number of potential US citizens present in northeast Syria who were in ISIS-held territory and now claim US citizenship. The Department is looking into these cases," a US State Department official told CNN.

A US official told CNN that the number of known people in northeast Syria claiming to be US citizens is "less than two dozen."

Generally there are many more self-proclaimed Americans than actual Americans among detained fighters and most of the people claiming citizenship haven't provided identifying documents to support their claim, a person with knowledge of the issue told CNN.

Earlier this year, the US government announced that one of those "affiliates," Hoda Muthana, would not be repatriated to the US, despite her pleas to return and face justice. Although she was born in New Jersey, the government said she was never a US citizen due to her father's diplomatic status.

Many of these "affiliates" are located at the Al Hol camp in Syria which has a current population of 73,041 people, 65% of whom are children under the age of 18, according to the UN.

The camp's population has grown by more than 60,000 since December and is made up of 43% Syrians, 42% Iraqis and 15% "other third country nationals."

The number of foreigners in detention has grown in recent weeks following the capture of the terror group's last area of territorial control in Baghouz, Syria, late last month.

The fate of the detained "foreign terrorist fighters" has been a major concern for US military planners as they seek to execute President Donald Trump's orders to substantially reduce the 2,000 US troops that are currently in Syria.

Senior US defense officials have long encouraged countries to repatriate their citizens but those efforts have had limited success as many countries, including European allies, have balked at repatriating their citizens.

Many countries are reluctant to do so because of the difficulty of prosecuting suspected ISIS members based on evidence collected on the battlefield.

The US has repatriated several of its citizens to face trial in US courts.

"The United States will continue to repatriate and, when appropriate, prosecute its citizens, as we have done in the past," a State Department official told CNN.

The US military has also facilitated the return of foreign terrorist fighters from Syria to Kazakhstan, Macedonia and Morocco.

US officials say additional fighters have been repatriated by several countries that have chosen not to make those repatriations public due to domestic political concerns.

Concerns over detention facilities

There are concerns that the detention facilities currently being used by the US-backed Syrian Democratic Forces are not built to house such large numbers of detainees for an extended period of time.

Earlier this month the Syrian Democratic Forces were compelled to respond to an attempted prison break at Dayrik prison in Syria. While the SDF were able to prevent any detainees from escaping, the US continues to be concerned about the long-term fate of the ISIS prisoners.

"Even though we would assess the SDF had done an absolutely admirable job in managing the situation, an idea of, in some cases, 60,000ish numbers of ISIS affiliates and fighters being held by a non-state actor. This means that they are going to need significant help," the senior defense official told CNN.

"Repatriating foreign terrorist fighters to their countries of origin is the best solution to prevent them from returning to the battlefield," Robertson, the Defense Department spokesman, told CNN.

And other senior US military leaders have expressed concerns that ISIS members have made a calculated decision to enter detention in the hopes of mounting a resurgence down the road.

"What we are seeing now is not the surrender of ISIS as an organization but a calculated decision to preserve the safety of their families and the preservation of their capabilities by taking their chances in camps for internally displaced persons and going to ground in remote areas and waiting for the right time to resurge," Gen. Joseph Votel, the then commander of US Central Command told the House Armed Services Committee last month.