Secretary of State John Kerry made another phone call to Russian Foreign Minister Sergey Lavrov seeking to clarify the intent of Moscow’s military buildup in Syria and warning that continuous aid to President Bashar Assad will only escalate the Syrian conflict, the State Department said on Tuesday,

State Department spokesman John Kirby said Kerry called Lavrov for the third time in the last 10 days. 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 in a written statement.”

Kerry also made it clear to Lavrov that the U.S. is committed to the coalition it created to fight ISIS in the Middle East and that Assad couldn’t be a “credible member” of that group, the statement said. 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,” the statement said.

The conversation came after Russian President Vladimir Putin defended its military assistance to Assad’s government and said it was impossible to defeat ISIS without cooperating with Damascus. Putin also urged other countries to join the cause.

Reuters, citing a statement from the Russian Foreign Ministry, reports Kerry and Lavrov also talked about the ongoing Ukraine crisis and steps to ease the escalating tension in North Korea. However, the statement didn’t go into further detail, according to Reuters.

Kerry is scheduled to travel to London this week to address the Syrian refugee crisis with British Foreign Secretary Philip Hammond and the foreign minister of the United Arab Emirates, Kirby said.

White House spokesman Josh Earnest said Tuesday Obama may reach out to Putin by phone within the coming days and wouldn’t rule out a meeting between the two leaders at the United Nations General Assembly later this month. However, the administration has made clear that Kerry is in the lead on conversations with Russia.

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 boost Assad is “doomed to failure.”

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," the spokesman added. "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 in Moscow.

The Associated Press contributed to this report.