WASHINGTON >> Secretary of State John Kerry on Tuesday 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 buildup in Syria, and warning that ongoing aid to President Bashar Assad will only prolong the Syrian conflict.

State Department spokesman John Kirby said Kerry called Foreign Minister Sergey Lavrov. Kerry "made clear that Russia’s continued support for President 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," the State Department said later in a written statement.

Kerry also told Lavrov that the U.S. is committed to the existing coalition it has created to fight the Islamic State group and that Assad could not be a "credible member" of that group, the statement said, adding that the U.S. would welcome a "constructive Russian role" in an effort which will only succeed if there is "a political transition away from Assad."

Kerry’s call came shortly after Russian President Vladimir Putin defended his military assistance to Assad’s government and said it’s impossible to defeat the Islamic State group without cooperating with Damascus and urged other countries to join the cause.

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, Kirby said.

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 United Nations General Assembly. However, administration officials made clear that Kerry was in the lead on conversations with Russia about the Syria.

Earnest said the administration’s stance on Russia’s moves in Syria remains the same as it was last week when Obama told U.S. 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 destabilizing 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."

The Obama administration has been perplexed 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 air base near the coastal town of Latakia, a stronghold of the Syrian president. But U.S. 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 longstanding 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."

At the Pentagon, spokesman Peter Cook said Defense Secretary Ash Carter had no plans to speak to his Russian counterpart. "Secretary Kerry is taking the lead with regard to discussions with Russia and we’re going to leave it at that," he said.

Last year, former Pentagon chief Chuck Hagel made several calls to Defense Minister Sergei Shoigu to express concerns about Russia’s annexation of Ukraine’s Crimea region. And Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, also called his counterpart.