The U.S. and Russia are ending an 18-month freeze in military-to-military relations and initiating talks about how to pursue "deconfliction" of the American and Russian forces that are now both involved in the Syrian civil war, a Pentagon official said Friday.

The moves comes as Russia builds a large military base in Syria with troops and aircraft, apparently with the aim to provide direct support to the embattled regime of Syrian President Bashar Assad, which the U.S. opposes.

A top U.S. commander said Wednesday that the Russian activity could put U.S. pilots at risk and vastly complicate the current American-led air campaign to defeat the Islamic State militants who control large swaths of both Iraq and Syria.

The Pentagon expects to begin a "military-to-military conversation about what is happening on the ground … to avoid any possible miscalculation or misunderstanding," said one senior defense official on Friday. 

The U.S. suspended military-to-military relations with Russia in March 2014 shortly after Moscow invaded and annexed the Crimea region of Ukraine.

"This morning, Secretary of Defense Ash Carter had a constructive conversation with the Russian Minister of Defense Sergei Shoygu on the situation in Syria," Pentagon spokesman Peter Cook said in a statement Friday.

"They agreed to further discuss mechanisms for deconfliction in Syria and the counter-ISIL campaign," Cook said, referring to the Islamic State by an alternative acronym.

The Russians requested the call through civilian diplomatic channels, and the call lasted about 50 minutes, the senior defense official said.

The new Russian base is coming together at an airport in Latakia, on Syria's Mediterranean coast. Construction includes reinforcing aircraft runways, building helicopter pads, installing mobile housing units and putting up several new buildings, including an air traffic control tower, according to publicly available satellite imagery.

Reports suggest the Russian military is arming the perimeter of the airfield with tanks and artillery.

During the phone call Friday morning, the Russian defense minister told Carter that the activities were "defensive in nature and designed to honor commitments made to the Syrian government," according to the senior defense official.

The Russians have committed to defending the Assad regime against rebel forces. President Obama has repeatedly called on Assad to step down and blamed him for the chaos in Syria and the rise of the Islamic State.

Army Gen. Lloyd Austin, chief of U.S. Central Command, told lawmakers on Capitol Hill on Wednesday that the Russian activities are troubling and "could increase the friction in that battle space significantly."

The buildup is one of Moscow's most aggressive military provocations in years and could be a vital lifeline for the Assad regime, whose forces have suffered a series of battlefield losses to Islamic State extremists and other rebel groups.

The U.S. and Russia both are eager to defeat the Islamic State militants in Syria but otherwise are backing different factions in the multipolar civil war.

The level of communication and cooperation that will emerge between the two militaries remains unclear.

Cooperation could include sharing tactical information about flight plans or troop movements to avoid in-air collisions or inadvertent strikes. The two militaries could coordinate strategic plans to defeat Islamic State militants. And they could discuss military aspects of a long-term political solution to the four-and-a-half-year-old civil war.

"The secretary and the minister talked about areas where the United States and Russia's perspectives overlap and areas of divergence," Cook, the Pentagon spokesman, said of the phone call between the two military leaders.

He added that Carter "emphasized the importance of pursuing such consultations in parallel with diplomatic talks that would ensure a political transition in Syria. He noted that defeating ISIL and ensuring a political transition are objectives that need to be pursued at the same time. Both the secretary and the minister agreed to continue their dialogue."

The Russian forces in Latakia are about 100 miles from Islamic State-held territory near Aleppo, an area where U.S. aircraft fly daily sorties and launch airstrikes against Islamic State targets on the ground.

The resumption of military-to-military conversations is likely welcome news to Gen. Mark Welsh, Air Force chief of staff.

Speaking Tuesday at the Air Force Association's annual conference outside Washington, D.C., Welsh acknowledged concerns about a "coordination problem" if Russian aircraft begin regular flights in Syria.

"It would be helpful to talk to the Russians about what they're going to do," Welsh said at the time.

Welsh noted as part of his comments at the Air Force Association's Air and Space Conference that Russia is closing the technological gap with the U.S., which complicates the question of what forces the Air Force could look to put in the region as a counter.

"They have a great capability in their air force and they've improved it tremendously in the last few years," Welsh noted. "I don't know exactly what they plan to put there. I don't know what the Russians' plans are, and I don't know what the counter move or the proposal will be from EUCOM to CENTCOM. Until we know that we're speculating in my mind."

Staff writer Aaron Mehta contributed to this story.