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".
He also announced that 475 US military personnel would be sent to Iraq but said they would not have a combat role.
"The US president has spoken directly about the possibility of strikes by the US armed forces against Isil (IS) positions in Syria without the consent of the legitimate government," ministry spokesman Alexander Lukashevich was quoted as saying.
"This step, in the absence of a UN Security Council decision, would be an act of aggression, a gross violation of international law."
Syria also repeated its warning that the US had to co-ordinate with the Syrian government before launching air strikes on its territory.
"Any action of any kind without the consent of the Syrian government would be an attack on Syria," National Reconciliation Minister Ali Haidar said on Thursday.
Last month Syria offered to help the US fight Islamic State, however the US ruled that out.
The US has launched more than 150 air strikes against the group in Iraq and has provided arms to Iraqi and Kurdish forces fighting against IS.