WASHINGTON — 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 Islamic State fight 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.

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 intentions are unclear, initial signs suggest a focus on providing air support to Syrian forces and humanitarian relief operations. The official, speaking on condition of anonymity, said the U.S. expects Russia to send ground forces to protect and support these operations.