US secretary of state John Kerry has made his third phone call to his Russian counterpart in the last 10 days, the State Department said, seeking to clarify the intent of Moscow's military build-up in Syria and warning that ongoing aid to President Bashar Assad will only prolong the Syrian conflict.
The State Department said Mr Kerry called Russia's Foreign Minister Sergey Lavrov and "made clear that Russia's continued support for Assad risks exacerbating and extending the conflict and undermining our shared goal of fighting extremism if we do not also remain focused on finding a solution to the conflict in Syria via a genuine political transition".
Mr Kerry also told Mr Lavrov that the US was committed to the existing coalition it has created to fight Isil and that Mr Assad could not be a "credible member" of that group, the statement said, adding that the US would welcome a "constructive Russian role" in an effort which will only succeed if there is "a political transition away from Assad".
Mr Kerry's call came shortly after Russian President Vladimir Putin defended his military assistance to Assad's government and said it was impossible to defeat Isil without co-operating with Damascus. He urged other countries to join the cause.
At the White House, spokesman Josh Earnest said President Obama might reach out to Mr Putin by phone in the coming days and would not rule out a meeting of the two leaders later this month at the United Nations General Assembly.
However, administration officials made clear that Mr Kerry was in the lead on conversations with Russia about the civil war in Syria.
Mr Earnest said the administration's stance on Russia's moves in Syria remained the same as it was last week when Mr Obama told US troops that a strategy to prop up Assad was "doomed to failure".
"We've made clear that further support, military or otherwise, for the Assad regime is destabilising and counter-productive, principally because Assad has lost the legitimacy to lead that country," he said, adding: "Russia's decision to double down on Assad is a losing bet."
President Assad has said he will not quit under foreign pressure, insisting that the Syrian people must decide.
Western powers and much of the Syrian opposition say it is not conceivable for Mr Assad to lead a post-war Syria.
Mr Assad said Iran was supporting his government "politically, economically and militarily" but denied that Iranian ground forces had been sent to Syria.
The latest comments come as Russia increases its presence in Syria.
Mr Assad said that the president of Syria "comes to power with the people's assent through elections and if he leaves, he leaves if the people demand it".
Mr Assad was re-elected in 2014 with 88.7pc of the vote.
However, the election only took place in government-held areas and the opposition said the vote had no credibility in the midst of a civil war.