US President Barack Obama said on Thursday it may take some months for Russia, Iran and the Syrian ruling elite to accept that there can be no end to Syria's civil war or a political settlement while President Bashar al-Assad remains in power.

Obama said that Moscow and Tehran recognised Islamic State as a "serious threat" but Russia's efforts in Syria were aimed at propping up Assad.

"Bottom line is, I do not foresee a situation in which we can end the civil war in Syria while Assad remains in power," he told reporters in Manila on the sidelines of the annual Asia-Pacific Economic Cooperation (APEC) summit.

"What we are doing with our coalition members is recognising that it may take some months for the Russians and the Iranians and frankly some members of the Syrian government and ruling elites within the regime to recognise the truths that I just articulated."

Russia began air strikes in Syria at the end of September. It has always said its main target is Islamic State militants, but most of its bombs in the past hit territory held by other groups opposed to its ally Assad.

Islamic State claimed responsibility for last week's killing spree by bombers and gunmen in Paris and for the bombing of a Russian airliner over Egypt last month.

Obama added that if he could get all the parties talking on the issue, "that could create space for that pivot".

The US president also said he could close the Guantanamo Bay prison in Cuba while keeping Americans safe, but acknowledged he would face tough resistance from Congress.

"I will guarantee you there will be strong resistance because in the aftermath of Paris, I think there is a very strong tendency for us to get worked up around issues that don't actually make us safe but make for good political soundbites, whether it is refugees or Guantanamo," he said.