The US military has been transferring dozens of captured ISIS foreign fighters from Syria to their countries of origin, according to multiple US defense officials -- however, hundreds of foreign fighters remain in detention in Syria.
US military aircraft are helping to return the captured fighters to their countries, according to two US defense officials.
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The US-led military coalition that's fighting ISIS has repeatedly declined to comment on the repatriation of foreign fighters.
"There's nothing more that I could tell you about any efforts to repatriate them. That's an issue for national governments," the deputy commander of Combined Joint Task Force-Operation Inherent Resolve, UK Maj. Gen. Felix Gedney, told reporters last week.
"What I can tell you, though, is that the Syrian Democratic Forces have done a remarkable job in capturing and continuing to detain these individuals that remain a threat globally, and specifically to our countries," Gedney said.
While many of the countries that have received detainees have chosen to keep quiet about the repatriations, the Pentagon confirmed on Tuesday that the Republic of Macedonia had taken custody of a group of foreign fighters.
"Today's transfer of Foreign Terrorist Fighters to their country of origin, Macedonia, marks a significant milestone in the much-needed cooperative effort to combat the global threat of terrorism," Pentagon spokesman Eric Pahon told CNN.
"The Department of Defense commends Macedonia for repatriating its citizens who have been detained by the Syrian Democratic Forces following their capture while fighting for ISIS," he added.
The repatriation of captured foreign fighters to their countries had been an ongoing concern among Pentagon officials as ISIS has lost territory in Syria and Iraq.
"We're gathering up hundreds now of detainees," Secretary of Defense Jim Mattis told reporters in February, following meetings in Europe with his counterparts on the detainee issue. "The important thing is that the countries of origin keep responsibility for them."
"It's an international problem -- it needs to be addressed and we're all engaged on doing that," Mattis added.
The US recently repatriated two American citizens from Syria, where they had been held by the Syrian Democratic Forces. Those two individuals are now being prosecuted by the Department of Justice.
Another individual, a dual US-Saudi national who was captured on the battlefield in Syria and is accused of having been a member of ISIS, remains in detention in Iraq. His fate has been the subject of a months-long legal battle between the US government and the American Civil Liberties Union.
Many countries have expressed unwillingness to take back foreign fighters amid fears that there might not be enough evidence to try them for their association with terrorism.
There are also concerns that former ISIS operatives could bring some of the tactics they learned on the battlefield back to their countries of origin.
"The government of France said that they don't want these people back," French Brig. Gen. Frederic Parisot, the director of civil-military operations for the coalition, told reporters last month.
The US-backed Syrian Democratic Forces currently hold hundreds foreign terrorist fighters.
An increase in detainees
Pentagon spokesman Navy Cmdr. Sean Robertson told CNN that "as of July 2018, there are nearly 600 foreign terrorist fighters from more than 40 countries in SDF custody."
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.
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.