"Pro-regime" forces that attacked a Syrian Democratic Forces (SDF) headquarters in northeastern Syria -- and were hit in return by U.S. airstrikes -- included Russian mercenaries, according to a Pentagon official. If Russians are among those who were killed, this would mark the first time a U.S. airstrike has killed Russians in Syria, CBS News' David Martin reports.

American advisers were present at the SDF headquarters. Pentagon officials, however, do not believe the Russians were after the Americans. Officials believe they were trying to seize a nearby oil field.

The U.S.-backed SDF, aided by coalition support, is battling Islamic State of Iraq and Syria (ISIS) militants east of the Euphrates River.

The U.S. airstrikes on Syrian government-backed troops Wednesday was a rare strike against forces that support Syrian President Bashar Assad's regime. Syrian government forces are active on the other side of the river around the city of Deir el-Zour.

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The U.S. coalition said in a statement that it launched the strikes in self-defense after as many as 500 attackers began what appeared to be a coordinated assault on an SDF headquarters, where Turkish forces are accompanied by U.S. troops.

U.S. officials said no Americans were injured or killed in the attack by the pro-regime forces. They spoke on condition of anonymity as details were still emerging on the attack.

Syrian state media said the U.S.-led coalition bombing left dozens killed and wounded.

Later Thursday, U.S. Defense Secretary James Mattis called the shootout with pro-regime forces "a perplexing situation," but he seems inclined to cut the Russians a break, Martin reported. Mattis said the situation is perplexing because he has "no idea why they would attack."

He said the attacking force was made up of about 300 troops with tanks and artillery. Mattis said the U.S. was still trying Thursday to establish exactly who the attackers were but added that the Russians have said they did not have forces there and "you can't expect somebody to deconflict something they can't control."

He also said he doesn't think there were any Russian casualties.

In retaliating with artillery and airstrikes, the coalition knocked out two tanks and some pro-regime artillery. By Thursday afternoon Eastern Time, the pro-regime unit had retreated across the Euphrates and was back on the right side of the deconfliction line, reported Martin.