A local fighter, a Kurd from the YPG militia, which dominates the Syrian Democratic Forces, was killed when an Isis unit fired what local commanders said was a heat-seeking missile at the group on Saturday morning.
The US-backed force known as the Syrian Democratic Forces announced that it captured five ISIS members in the extremist group last stronghold in Deir Ezzor province, east of the Euphrates river, bordering Iraq.
In a press release on Sunday, SDF confirmed capturing the five ISIS members, saying that "They were preparing to launch attacks against the fleeing civilians from that areas" pointing out that the capture happened on December 30. Also captured were two Pakistani fighters and an Irish man.
Officials with Operation Inherent Resolve, the US -led coalition against ISIS militants in Iraq and Syria, were aware of the SDF release, but couldn't confirm the account and were investigating, Stars and Stripes reported.
Pentagon spokesman Sean Robertson told U.S. media: "We are aware of open source reports of reportedly American citizens now in custody who were believed to be fighting for Isis".
The injuries would be the first known casualties sustained by Western anti-IS coalition forces since President Donald Trump announced in late December his intention to pull out some 2,000 USA troops in Syria, declaring that IS has been defeated. The 35-year-old, who reportedly went by the alias Abu Zaid al-Ameriki.
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The Americans were identified as Warren Christopher Clark (aka Abu Mohammad al-Ameriki) of Houston, and Zaid Abed al-Hamid (aka Abu Zaid al-Ameriki), originally from the US, but no specific location was reported.
The dual US-Saudi national was held by the US military in Iraq without being tried since September 2017. Of the 64 who were named, the whereabouts or status of 28 were publicly unknown, more than 20 were believed to have died overseas, 12 were apprehended and three had returned to the USA without facing charges.
It's not yet clear what will happen to the American prisoners, but it isn't the first time the SDF has caught US citizens on the battlefield.
Clark was identified in an NBC News report last February as an American who had sent his resume to ISIS, in hopes of joining the terror group.
In November, a court filing shows that the Defense Department released him in Bahrain following negotiations with the ACLU.