MOSCOW/PARIS (Reuters) - Russia said on Thursday air strikes against Islamist militants in Syria without a U.N. Security Council mandate would be an act of aggression, raising the possibility of a new confrontation with the West in coming weeks.

"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," Foreign Ministry spokesman Alexander Lukashevich said.

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

Barack Obama said on Wednesday he had authorized U.S. air strikes for the first time in Syria and more attacks in Iraq, in an escalation of the campaign against the Islamic State militant group, which has taken control of large areas of both countries.

Western states have ruled out working with Syrian President Bashar al-Assad, saying he has indirectly helped Islamic State grow in order to weaken other opposition groups.

Obama, who is due to host a leaders' security conference at the U.N. General Assembly in two weeks' time, made no mention of seeking an international mandate for action in Syria.

Russia, a veto-wielding permanent member of the Security Council, has given Assad crucial backing in Syria's civil war, which has killed more than 200,000 people.

It has provided arms and blocked Western and Arab-backed efforts to adopt Security Council resolutions condemning him or threatening him with sanctions.

It has repeatedly argued that it does not believe the Syrian opposition can fill the void that would be left by Assad's departure, warning the country would fall into the hands of Islamic militants.

France, a key ally for the United States in the planned coalition, said on Wednesday it was ready to take part in air strikes in Iraq, but said its involvement in any military action in Syria would need to have international law behind it.

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Foreign Minister Laurent Fabius said the Iraqi government has asked for help internationally, but in Syria the legal basis would have to be established first.

French officials have said that would come either through a Security Council resolution or under Article 51 of the U.N. charter, allowing for protection of threatened populations.

"The Russians aren't beholden to Assad," said a senior French diplomat.

"It's in their interest as much as ours to fight terrorism so we can hopefully find some pragmatic and objective ways to resolve our differences and find a way to agree."

(Reporting by Alexei Anishchuk and John Irish; Editing by Timothy Heritage and Andrew Roche)