US Secretary of State John Kerry has told Russian Foreign Minister Sergey Lavrov that Moscow's "continued support for Syrian President Bashar al-Assad risks exacerbating and extending the conflict" in the country.
Kerry made the comments on Tuesday in his third phone call to Lavrov in the past 10 days, a US state department official said, seeking to clarify the intent of Russia's military build-up in Syria.
The call came shortly after Russian President Vladimir Putin defended his military assistance to Assad's government and said it was impossible to defeat the Islamic State of Iraq and Levant (ISIL) without cooperating with Damascus and urged other countries to join the cause.
A readout of the call released by the state department late on Tuesday, said: "Secretary Kerry also reaffirmed the US commitment to fight ISIL with a coalition of more than 60 countries, of which Assad could never be a credible member, and emphasised the US would welcome a constructive Russian role in counter-ISIL efforts.
"The secretary stressed that there is no military solution to the overall conflict in Syria, which can only be resolved by a political transition away from Assad."
'Doomed to failure'
Kerry will travel to London later this week for talks to include the situation in Syria with British Foreign Secretary Phillip Hammond and the foreign minister of the United Arab Emirates.
OPINION: Could Syria be Putin's Afghanistan?
At the White House, spokesman Josh Earnest said Obama might reach out to Putin by phone in the coming days and would not rule out a meeting of the two leaders later this month at the UN General Assembly.
However, administration officials made clear that Kerry was in the lead on conversations with Russia about Syria.
Earnest said the administration's stance on Russia's moves in Syria remains the same as it was last week when Obama told US troops that a strategy to prop up Assad is "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.
"Russia's decision to double down on Assad is a losing bet."
Earnest renewed calls for Russia to cooperate with the US-led coalition conducting air strikes against ISIL which controls large swaths of Syria and Iraq.
RELATED: Syrian refugees: A catastrophe in numbers
"The Russians indicate that they share that goal, and we'd like to see them work cooperatively with the rest of the international community to advance it," he said.
The Obama administration has been unsettled by Russia's ramped up support for Assad, which includes about a half-dozen battle tanks delivered in recent days.
Moscow has also sent other weaponry, along with military advisers, technicians, security guards and portable housing units, with the apparent goal of setting up an airbase near the coastal town of Latakia, a stronghold of the Syrian president.
But US officials say Putin's intentions in Syria, particularly in the medium- to long-term, remain a mystery.
Earnest told reporters that "this would not be the first situation in which President Putin's motivations are rather hard to discern".
"The decision-making process in that country is rather opaque," Earnest said of Russia, adding that Moscow has long used Syria as a "client state".
"That long-standing client-state relationship might lead one to conclude that President Putin is factoring into this equation some long-term considerations," he said.
"But it's not clear exactly what he believes is the best way to advance those longer-term interests that Russia may have inside of Syria."
Source: Al Jazeera and agencies