U.S. President Barack Obama said Russia must make a strategic decision about Syria and the next several weeks will show whether Russian President Vladimir Putin will give up backing the Syrian regime of Bashar Assad to join in a broad campaign against Islamic State.

The U.S. won't in any circumstances agree to a political settlement for the civil war in Syria that leaves Assad in power because he's lost all legitimacy, Obama said. As long as Assad remains, there is no way to unite the country's various factions for the fight against Islamic State.

"It is not conceivable that Mr. Assad can regain legitimacy in a country in which a large majority of that country despises Assad, and will not stop fighting so long as he's in power," Obama said Sunday at a news conference in Kuala Lumpur.