This undated file image posted on a militant website on January 14, 2014, which has been verified and is consistent with other AP reporting, shows fighters from the al-Qaida linked Islamic State in Raqqa, Syria.
There was no immediate comment from the Syrian government or from Moscow.
The US-led Coalition told Rudaw English that as of Tuesday, SDF partner forces "have liberated approximately 42,000 square-kilometers in Syria" which is estimated to be "77 percent of the territory previously held by ISIS".
Al-Sinaaiya is an industrial area northeast of the city of Deir Ezzor, about seven kilometres (four miles) from the east bank of the Euphrates. Rich in oil, it is bisected by the Euphrates River and abuts Iraq.
The captured terrorist is not the only example for US nationals joining Daesh. Talks have been underway to extend a formal demarcation line that has separated the campaigns, officials have said.
On Friday, he warned the army and its allies against firing across the river, which he said they had done in recent days.
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Several fighters in the Syrian Democratic Forces (SDF) were wounded and required medical care.
"I don't see them as good Muslims", Khweis said of ISIS. "We do not allow the regime or its militias to cross to the eastern riverbank".
Syrian troops and allied forces captured villages on the western bank of the Euphrates on Saturday, state media said. "We are moving forcefully and quickly".
Earlier, Colonel Ryan Dillon, a spokesman for the US-led anti-IS coalition, said the SDF "has taken note" to ensure that any fighters they capture are brought to "the proper authorities". The strike occurred as both Russian and coalition forces converged on the former ISIS stronghold of Deir Ezzor.
Coalition military forces, he said, conducted three strikes consisting of 13 engagements against ISIS targets on September 14 in Huwayjah and Rawah. "The battles are continuous", he said.