U.S.-led coalition forces are looking into whether Russian contractors were involved in an attack against Syrian Democratic Forces on Wednesday.

U.S. Central Command reported Wednesday that forces supporting Syrian President Bashar Assad conducted an attack on Syrian rebels allied with the U.S., prompting retaliation strikes from coalition forces.

CNN reported Thursday afternoon that U.S. forces are now looking into whether Russian contractors were involved in the initial attack against U.S.-allied forces, which did not lead to any American casualties.

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A U.S. official told the news outlet that it was unclear yet whether Russian operators in the region were involved, and that the U.S. had also not ruled out the possibility of the involvement of Iranian-backed forces.

The attack took place in the Middle Euphrates River Valley 8 kilometers east of a negotiated "de-confliction line" meant to separate Russian-backed government forces and U.S.-backed rebels, according to Central Command.

The U.S. maintains about 2,000 troops in the country. In January, Secretary of State Rex Tillerson Rex Wayne TillersonGary Cohn: 'I haven't made up my mind' on vote for president in November Kushner says 'Alice in Wonderland' describes Trump presidency: Woodward book Conspicuous by their absence from the Republican Convention MORE outlined a plan for U.S. policy in Syria, stating that a full troop withdrawal would be disastrous for the country.

"Total withdrawal would restore Assad and continue brutal treatment of his own people," Tillerson said last month.

“Assad’s regime is corrupt, and his methods of governance and economic development have increasingly excluded certain ethnic and religious groups,” Tillerson added. “Such oppression cannot persist forever.”