US President Barack Obama said Tuesday he expected a gradual shift in Russia's strategy on Syria as it counted the cost of keeping President Bashar al-Assad in power.
"I think it is possible over the next several months that we both see a shift in calculation in the Russians and a recognition that it's time to bring the civil war in Syria to a close," Obama said on the sidelines of a UN climate summit in Paris.
"It's not going to be easy. Too much blood has been shed," he said, adding that Russia had invested years in keeping Assad's regime in place.
But he added that the bombing of a Russian passenger plane over Egypt by the Islamic State group last month, and last week's downing of a fighter jet by Turkey, was slowly changing Putin's calculations.
Signup to our newsletter and follow us on Facebook and Twitter!
"I think Mr Putin understands that, with Afghanistan fresh in the memory, for him to simply get bogged down in an inconclusive and paralysing civil conflict is not the outcome that he's looking for," said Obama, referring to the Afghan conflict of the 1980s which became a major drain on Moscow's resources.
Obama acknowledged the sharp difference that remains over the future of Assad, but he said Russia would eventually agree that the Syrian leader had to go.
"I consider somebody who kills hundreds of thousands of his own people illegitimate," Obama said in reference to Assad.
"But regardless of the moral equation, as a practical matter it is impossible for Mr Assad to bring that country together, and to bring all the parties into an inclusive government."
The US president said the next step in diplomatic efforts in Vienna was to include moderate opposition groups -- "some of which, frankly, you know, we don't have a lot in common with, but do represent significant factions inside of Syria".
"Ultimately Russia's going to recognise the threat that ISIL (the Islamic State group) poses to its country, to its people, is the most significant and that they need to align themselves with us who are fighting ISIL," he added.