(CNN) The US-backed Syrian Democratic Forces currently hold nearly 600 foreign terrorist fighters.
"As of July 2018 there are nearly 600 Foreign Terrorist Fighters from more than 40 countries in SDF custody," Pentagon spokesman Navy Cdr. Sean Robertson told CNN.
Robertson added that the US military was also aware of more than 400 Syrian nationals who were being detained by the Syrian Democratic Forces.
The apparent increase in detainees -- the Pentagon had previously put the number at more than 400 -- reflects recent gains the Syrian Democratic Forces have made against ISIS in its last holdouts in the Middle Euphrates River Valley and along the Syria-Iraqi border.
A US military official with the coalition fighting ISIS told CNN that of the foreign terrorist fighters in detention, about 40 are from Russia, about a dozen are from Germany and a similar number are from France.
The New York Times was the first to report the increase in detainees.
The repatriation of captured foreign fighters to their countries had been an ongoing concern among Pentagon officials as ISIS lost territory in Syria and Iraq.
Senior American defense officials have sought to convince their international counterparts to repatriate captured foreign fighters.
"We're gathering up hundreds now of detainees. The important thing is that the countries of origin keep responsibility for them," Secretary of Defense Jim Mattis told reporters in February following meetings in Europe with his counterparts on the detainee issue.
"It's an international problem -- it needs to be addressed and we're all engaged on doing that," Mattis added.
However, many countries have expressed an unwillingness to take back foreign fighters amid fears that there could not be enough evidence to try them for their association with terrorism.
There are also concerns that these former ISIS operatives could bring some of the tactics they learned on the battlefield back to their countries of origin.
US officials have cited concerns about the capacity of the Syrian Democratic Forces to detain so many fighters over a long period.
"Our Syrian Democratic Forces partners have adequate facilities right now to house and care for detainees. As you can imagine, it is a drain on their resources. They are not a policing organization," US Army Col. Thomas Veale, a spokesman for the coalition, said last month.
While the US has provided funds to the Syrian Democratic Forces to help improve detention facilities, US and coalition forces are not involved in detention operations.
Veale said "several countries" had expressed willingness to repatriate their citizens but he did not specify how many or identify them.