LONDON — As the first Russian combat aircraft arrived in Syria, 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.

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 Syria risked an escalation of the civil war there or even an inadvertent confrontation with the United States. Last week, President Obama condemned Russia’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 Russia might yet be drawn into a more constructive role in resolving the four-year-old civil war.

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.