American and Russian military chiefs began talks Friday over Moscow's buildup in Syria, signaling the U.S. is resigned to Russia's emerging plans but anxious to avoid dangerous misunderstandings on the battlefield.

A 50-minute phone call between U.S. Defense Secretary Ash Carter and Russia Defense Minister Sergei Shoigu marked the first military-to-military conversation between the two countries in more than a year. And it came as Russia continued to send aircraft, troops and military equipment into Syria and the U.S.-led coalition kept up its assault against Islamic State militants.

Russian Foreign Minister Sergey Lavrov broached the idea of direct talks earlier this week in a phone call with U.S. Secretary of State John Kerry, and Shoigu initiated the call to Carter on Friday.

U.S. officials are increasingly worried that Russia's plan to defend and support the government of Syrian President Bashar Assad could conflict with coalition airstrikes and other military operations against the Islamic State. The main focus of the phone call, according to a senior defense official, was to talk about how the fight against IS will go forward without miscalculations.

Russia has called for a broad coalition to fight the Islamic State group and has indicated that helping Assad's military is the best way to do that. However, U.S. policy has centered for the duration of the civil war in Syria on the idea that Assad must step down to make way for a new government. Until recently, Russia had seemed to agree with that policy.

The U.S. has been concerned about how to respond to Russia's increased support for Assad and how to interpret the intent behind the delivery of tanks, helicopters and other military equipment to a base in the coastal province of Latakia.

Defense Secretary Ash Carter speaks at the Pentagon during a news conference, Friday, May 1, 2015, to discuss the Defense Department's annual report on sexual assault in the military. AP Photo/Manuel Balce Ceneta

A senior U.S. official said Shoigu told Carter that Russia's increased military activities are defensive in nature, and designed to honor Moscow's commitments to the Assad government.

A U.S. intelligence official said that while Moscow's ultimate intentions are unclear, initial signs suggest a focus on providing air support to Syrian forces and to humanitarian relief operations. The official said the U.S. expects Russia to send ground forces to protect and support these operations.

The officials were not authorized to discuss the issue publicly and spoke only on the condition of anonymity.

On Friday, a spokesman for Russian President Vladimir Putin said Russia would consider sending troops to fight inSyria if Damascus requested them.

Kerry, meanwhile, in comments to a London television station, said the U.S. campaign against the Islamic State will continue unabated, "and we need to make sure those don't conflict with Russia's efforts against ISIL."

Secretary of State John Kerry (L) testifies before the Senate Armed Services Committee about the nuclear deal struck between Iran and six nations, including the United States, with fellow cabinet secretaries on Capitol Hill July 29, 2015 in Washington, DC. Chip Somodevilla/Getty Images

"We welcome their help to get rid of ISIL and to have a legitimate political transition," Kerry said. "But you will not end the crisis of Syria as long as Assad remains this magnet for people to come in and fight. We have to find a way forward on a political transition."

In the Carter-Shoigu phone conversation, Pentagon press secretary Peter Cook said, the U.S. defense secretary "emphasized the importance of pursuing such consultations in parallel with diplomatic talks that would ensure a political transition in Syria."

U.S.-Russian talks about the need for a peaceful political resolution in Syria have been occurring intermittently since 2012. Recent efforts have also included Saudi Arabia.

The U.S. suspended military relations with Russia last year over the Russian invasion and annexation of Ukraine's Crimea region. It's not clear if Gen. Martin Dempsey, chairman of the Joint Chiefs of Staff, or other U.S. military leaders will also reach out to their counterparts.

An injured man walks amid debris as residents reacts at a site hit by what activists said were barrel bombs dropped by forces of Syria's President Bashar al-Assad, at al-Thawra neighborhood in Idlib city April 20, 2015. REUTERS/Ammar Abdullah

Russian Defense Ministry spokesman Maj. Gen. Igor Konashenkov issued a statement noting that "the course of the conversation has shown that the sides' opinions on the majority of issues under consideration are close or coincide. The ministers noted the restoration of contacts between the countries' defense ministries and agreed to continue consultations. "

"This (Syria) crisis has got to be solved," Kerry said before meeting with the United Arab Emirates Foreign Minister Abdullah bin Zayed in London.

After his third phone call with Lavrov since Labor Day weekend, Kerry said the Russian proposed talks among the countries' military leaders "to discuss the issue of precisely what will be done to de-conflict with respect to any potential risks that might be run and have a complete and clear understanding as to the road ahead and what the intentions are."

"It is vital to avoid misunderstandings, miscalculations (and) not to put ourselves in a predicament where we are supposing something and the supposition is wrong," Kerry said.

He said Lavrov had told him Russia was interested only in confronting the threat posed by the Islamic State group in Syria but "I am not taking that at face value."