London -- America's Kurdish allies in Syria, currently fretting over when the Trump administration will pull out the 2,000 U.S. troops who guarantee their safety, said Wednesday that they had apprehended an American teenager fighting with ISIS. If confirmed, the teen would be at least the second U.S. national held by the Kurds in Syria, among hundreds of captured foreign fighters whose fate is a major concern for U.S. officials trying to implement President Trump's order to withdraw American forces from the country.

The Kurdish militia known as the YPG, which has been the U.S. military's most valuable ally in the fight against ISIS in Syria, said eight foreign fighters were caught on Sunday and Monday in eastern Syria, about 15 miles from the Iraqi border.

An ISIS suspect identified by the U.S.-allied Kurdish YPG militia as 16-year-old U.S. national Soulay Noah Su is seen in a photo posted to social media by the YPG on Jan. 9, 2019.

According to the YPG, a 16-year-old boy from the U.S. identified as Soulay Noah Su was among the alleged terrorists who were trying to carry out an attack on civilians. They said the other captured militants were from Russia, Germany, Ukraine and central Asia.

Get Breaking News Delivered to Your Inbox

U.S. officials have not confirmed that the Kurds are holding an American teenager in Syria.

CBS News senior national security correspondent David Martin reported in December, after President Trump announced his intention to pull all U.S. forces out of Syria, that the Kurdish forces are holding an estimated 700 foreign combatants. The other known American suspect is a Houston man, Warren Christopher Clark.

Eight terrorists of US, Russian, German, Ukrainian, Tajik, Kazakh and Uzbek origins were captured by our units.https://t.co/N24lRy2Xuo pic.twitter.com/0DjA7Dj4hl — Rojava Defense Units | YPG (@DefenseUnits) January 9, 2019

The U.S. has been working with allies for months to figure out what to do with all the suspected ISIS militants in Kurdish custody. As CBS News Radio correspondent Cami McCormick reported in December, then-Defense Secretary James Mattis tried unsuccessfully to convince countries to take back their nationals. Part of the problem for the ISIS fighters' home countries are the legal ramifications of holding citizens who were picked up on the battlefield -- and how to prosecute them.

"It's one thing for the government to be very confident that an individual joined or tried to join ISIS," Joshua Geltzer, a senior counterterrorism official in the Obama administration, told the Associated Press this week. "Sometimes it's still another thing for the government to be able to mount confidently a criminal prosecution against that individual."

The Kurds warned last month that if U.S. forces withdraw from Syria, they might have to turn their attention to fending off an attack from neighboring Turkey, which considers the YPG a terrorist organization, and in turn release the prisoners.

The fear is not only that those foreign fighters could help ISIS rebuild in Syria and Iraq without a U.S. military presence, but that they could return to their own countries and launch attacks.