Russia on Thursday bombed US-backed fighters in southern Syria, according to a US official in Washington, who said the aggressive action by Moscow raises "serious concern."

"Today, Russian aircraft conducted a series of airstrikes near al-Tanf against Syrian Counter-ISIL forces that included individuals who have received US support," said the official, who requested anonymity.

"Russian aircraft have not been active in this area of southern Syria for some time, and there were no Syrian regime or Russian ground forces in the vicinity," the official said.

"Russia's latest actions raise serious concern about Russian intentions. We will seek an explanation from Russia on why it took this action and assurances this will not happen again