“The president believes that a mil-to-mil conversation is an important next step,” Secretary of State said on Friday in London, where he was visiting to consult with allies. “It will help to define some of the different options that are available to us.”

The Russian military buildup in Syria could serve the Kremlin’s interests in several ways. It could help strengthen Mr. Assad, whom Russia has long backed and who has suffered a number of military reversals in recent months. It could put Moscow in a stronger position to shape the formation of a new Syrian government if Mr. Assad is pushed out of power. It also helps Russia cement its strategic interests in what experts say is its most important new Middle East military outpost in decades.

But while Mr. Carter’s initial military-to-military talks were limited in scope, officials indicated that the larger goal was to draw the Russians into a political process that would ultimately replace Syria’s government of President , a longtime ally of the Kremlin.

Still, the stakes have become even higher, as a senior United States official on Friday confirmed that four Russian Su-27 fighter aircraft had been deployed to the air base in recent days, along with four large Hip troop-transport helicopters and four Hind helicopter gunships. The official, who spoke on the condition of anonymity to discuss intelligence reports, said that more than 20 Condor transport plane flights had delivered weapons and equipment to the air base in the past 10 days.

At Mr. Obama’s instruction, Defense Secretary Ashton B. Carter on Friday opened a dialogue on Syria with his Russian counterpart, Defense Minister Sergei K. Shoigu, aimed at making sure that American and Russian forces avoid running into each other by mistake. The Russians have sent tanks, other equipment, marines and now combat aircraft to their new military hub near Latakia in western Syria. The Americans have flown hundreds of air missions in Syria striking the Islamic State, also known as ISIS or ISIL.

But while Mr. Carter’s initial military-to-military talks were limited in scope, officials indicated that the larger goal was to draw the Russians into a political process that would ultimately replace Syria’s government of President Bashar al-Assad, a longtime ally of the Kremlin.

Secretary of State John Kerry, during an appearance with the foreign minister of the United Arab Emirates in London on Friday, said he hoped military talks with Russia on Syria would take place soon.

LONDON — As the first Russian combat aircraft arrived in , the Obama administration reached out to Moscow on Friday to try to coordinate actions in the war zone and avoid an accidental escalation of one of the world’s most volatile conflicts.

LONDON — Secretary of State said on Friday that the United States was prepared to engage in military-to-military talks with concerning .

“The president believes that a military-to-military conversation is an important next step,” Mr. Kerry said, “and I think, hopefully, it will take place very shortly.”

The initial purpose of the talks with Russia, Mr. Kerry said in London, will be to help “define some of the different options that are available to us as we consider next steps in Syria.”

Shortly after Mr. Kerry spoke, the Pentagon announced that Defense Secretary Ashton B. Carter had spoken by telephone with Sergei K. Shoigu, the Russian minister of defense. It was Mr. Carter’s first discussion with his Russian counterpart since he became defense secretary seven months ago. The two men agreed to continue discussions on “mechanisms for deconfliction” in Syria, Peter Cook, the Pentagon press secretary, said in a statement.

Mr. Cook described the discussion as “constructive” and said the two men had “talked about areas where the United States and Russia’s perspectives overlap, and areas of divergence.”

“The president believes that a mil-to-mil conversation is an important next step,” Mr. Kerry said, “and I think, hopefully, will take place very shortly.”

The diplomatic initiative amounted to a pivot for the Obama administration, which just two weeks ago delivered a stern warning to the Kremlin that its military buildup in risked an escalation of the civil war there or even an inadvertent confrontation with the United States. Last week, President Obama condemned ’s move as a “strategy that’s doomed to failure.”

But the White House seemed to acknowledge that the Kremlin had effectively changed the calculus in Syria in a way that would not be soon reversed despite vigorous American objections. The decision to start talks also reflected a hope that might yet be drawn into a more constructive role in resolving the four-year-old civil war.

Mr. Kerry said that the Obama administration would not change its basic goals in Syria: The defeat the Islamic State, also known as ISIS or ISIL, and a political solution for the conflict there.

The initial purpose of the talks with Russia, Mr. Kerry said, will be to help “define some of the different options that are available to us as we consider next steps in Syria.”