Obama says Bashar al-Assad must go to end Syria's civil war

US President Barack Obama on Thursday said Syria's civil war would not end unless Bashar al-Assad leaves power, discounting suggestions the Middle Eastern leader could take part in future elections.

"I do not foresee a situation in which we can end the civil war in Syria while Assad remains in power," Obama said on the sidelines of a trade summit in Manila.

Assad's fate has become a key stumbling block to peace in Syria and a point of contention between the West and Assad's backers in Moscow and Tehran.

Obama's comments come days after meeting Assad's top backer, Russian President Vladimir Putin, which appeared to see the two sides edge closer to an agreement.

Russia, seeking to maintain its foothold in Syria, has strongly opposed Assad's ouster.

But Obama insisted Syrians would not accept Assad staying in power, after a brutal civil war that has seen his regime carry out indiscriminate attacks on civilians.

"Even if I said that was okay, I still don't think it would actually work," Obama said.

"You could not get the Syrian people -- the majority of them -- to agree to that kind of outcome."

Ending the four year war in Syria has taken on increasing urgency as the Islamic State group has gained more and more territory, assets and recruits.

The militants are suspected of carrying out large-scale terror attacks in Paris, Beirut and the bombing of a Russian airliner, killing hundreds of civilians.