Russia has warned that US air strikes against militants in Syria would be a “gross violation” of international law.
A Russian foreign ministry spokesman said any such action, without the backing of the UN, would be “an act of aggression”.
It comes as US Secretary of State John Kerry meets Arab leaders in Saudi Arabia to try to build a coalition against Islamic State (IS) militants.
President Obama has threatened action against IS in Syria as well as Iraq.
IS controls large parts of Syria and Iraq after a rapid military advance.
In a speech outlining his strategy, Mr Obama said any group that threatened America would “find no safe haven”.