American officials have warned of a grim toll if Mr. Putin proceeds with a military invasion of Ukraine, including the potential deaths of 25,000 to 50,000 civilians, 5,000 to 25,000 members of the Ukrainian military and 3,000 to 10,000 members of the Russian military. Mr. Sullivan said on Friday that officials believed that an attack would likely start with missile and aerial attacks, and continue with a ground invasion.

On Friday, Mr. Biden spoke with other trans-Atlantic leaders on a call that included Mr. Scholz; Prime Ministers Boris Johnson of Britain, Mario Draghi of Italy and Justin Trudeau of Canada; and Presidents Macron, Andrzej Duda of Poland, Klaus Iohannis of Romania, Ursula von der Leyen of the European Commission and Charles Michel of the European Council; and the NATO secretary general, Jens Stoltenberg.

The leaders met for about 80 minutes, in a call that was initially supposed to be centered around xe2x80x9cdiplomacy and deterrence,xe2x80x9d the White House said.

Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff, also spoke on Friday with his Russian counterpart as well as allied chiefs of defense about the crisis in Ukraine, military officials said.

The calls came as Russia builds up its forces in Belarus, western Russia and Crimea. Ukraine said on Friday that Russian-backed separatists were holding military exercises in the slice of eastern Ukraine they controlled, at the same time that Russia holds exercises near Ukraine.

The drills tested the separatistsxe2x80x99 preparation for live-fire operations, practicing xe2x80x9cdriving artillery, tanks and armored vehiclesxe2x80x9d in field exercises, the Ukrainian statement said. Some units of the force, believed to number 30,000 troops, were put on their highest level of alert, the Ukrainians said, and senior Russian military officers were observing the activity.

The assessment from Kyiv was the latest evidence of a shift by officials there to more alarming commentary about the military risk facing the country. That follows weeks of efforts to minimize the threat of an invasion, seeking to calm the public, limit the economic fallout and avoid anything that could be deemed a provocation by Moscow.