Russia's Foreign Ministry said on Thursday airstrikes against Islamist militants in Syria without a UN Security Council mandate would be an act of aggression, Interfax news agency reported.

"The U.S. president has spoken directly about the possibility of strikes by the U.S. armed forces against ISIL positions in Syria without the consent of the legitimate government," ministry spokesman Alexander Lukashevich said.

"This step, in the absence of a UN Security Council decision, would be an act of aggression, a gross violation of international law."