WASHINGTON — The Biden administration will work with allies to transfer Soviet-made tanks to bolster Ukrainian defenses in the country’s eastern Donbas region, a U.S. official said on Friday.

The decision to act as an intermediary to help transfer the Soviet-made tanks, which Ukrainian troops know how to use, comes in response to a request from President Volodymyr Zelensky of Ukraine, the official said. It marks the first time in the war that the United States has helped transfer tanks.

The official said the transfers would begin soon, but declined to say how many tanks would be sent, or from which countries they would come. They will allow Ukraine to conduct long-range artillery strikes on Russian targets in Donbas, said the official, who spoke on the condition of anonymity because he was not authorized to speak publicly.

The tanks’ arrival could be another signal of a new phase in the war, which is five weeks old and has been dominated by Russian attacks on Ukrainian cities and installations from the air, and a stalled Russian advance on the ground. Earlier this week, Russian officials indicated that they were refocusing their efforts on eastern Ukraine, where Russian-backed separatists have been fighting Ukrainian soldiers since 2014.

Mr. Zelensky called on Sunday for NATO allies to provide tanks and planes, in addition to the antitank and antiaircraft weaponry that have become a staple of the arms transfers to Ukraine from the West. Frustrated at what he views as a slow pace of weapons transfers, Mr. Zelensky asked specifically for tanks, in remarks a day after President Biden met with senior Ukrainian officials in Poland.

An angry Mr. Zelensky criticized the West for what he called its “Ping-Pong” about weapons transfers. “I’ve talked to the defenders of Mariupol today,” he said, in a reference to the besieged city that has been under an onslaught from Russia for four weeks. “If only those who have been thinking for 31 days on how to hand over dozens of jets and tanks had 1 percent of their courage.”

In the past, the Biden administration has taken pains to call the weapons it is providing to Ukraine defensive, and has focused on smaller, easily portable arms. But as the war has progressed, the definition of defensive has become more elastic.

Ukraine had already found one source of tanks, capturing at least 161 from Russia on the battlefield, according to the military analysis site Oryx, though Russia has also destroyed a number of Ukrainian tanks. For its part, Russia has captured 43 Ukrainian tanks, according to analysts who study photos and videos on social media.

The decision to help transfer the tanks comes as the Ukrainian military has continued to turn back Russia’s ground advance. Pentagon officials have been quick to point out that Russia’s pivot to Donbas and away from capturing Kyiv, the capital, might be a necessity for Moscow after Russian forces stalled out in the central part of the country.

On Wednesday, Biden administration officials, citing declassified U.S. intelligence, said that President Vladimir V. Putin of Russia had been misinformed by his advisers about the Russian military’s problems in Ukraine. The intelligence, American officials said, also showed what appeared to be growing tension between Mr. Putin and his defense minister, Sergei K. Shoigu, who was once among the most trusted members of the Kremlin’s inner circle.

Russian officials have disputed the allegations, with the Kremlin on Thursday calling it a “complete misunderstanding” of the situation that could have “bad consequences.”