WASHINGTON — The Biden administration warned on Friday that with President Vladimir V. Putin of Russia moving more troops toward the Ukrainian border, a full-scale invasion of Ukraine could start before the end of the Winter Olympics in Beijing, despite speculation that the Russians would wait to invade.
“We continue to see signs of Russian escalation, including new forces arriving at the Ukrainian border,” Jake Sullivan, the president’s national security adviser, told reporters on Friday. Mr. Sullivan said that an invasion could begin “during the Olympics,” which are scheduled to end next week, and warned that all Americans should leave Ukraine in the next 24 to 48 hours.
“We are ready either way,” Mr. Sullivan said. “Whatever happens next, the West is more united than it has been in years.”
U.S. intelligence officials had initially thought Mr. Putin was prepared to wait until the end of the Winter Olympics in Beijing to avoid antagonizing President Xi Jinping of China, a critical ally. In recent days, they say, the timeline began moving up, an acceleration that Biden administration officials began publicly acknowledging on Friday.
Antony J. Blinken, the secretary of state, told reporters at a news conference in Melbourne on Friday that “invasion could begin at any time. And, to be clear, that includes during the Olympics,” which are scheduled to end on Feb. 20. Mr. Blinken added that U.S. officials “continue to see very troubling signs of Russian escalation, including new forces arriving at the Ukrainian border.”
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. On Friday, Mr. Sullivan said that officials believed that an attack would likely start with missile and aerial attacks, and continue with a ground invasion.
“Russia could choose in very short order to commence a major military action against Ukraine,” Mr. Sullivan said, but added that officials could not be sure exactly when, or if, Mr. Putin may decide to invade.
“The risk is now high enough and the threat is now immediate enough that this is what prudence demands,” he said. “We believe he very well may give the final go order,” he added, “but we are not standing here before you today saying the order has been given.”
The United States and many other countries have issued a series of increasingly urgent calls for their citizens to leave Ukraine.
On Thursday evening, President Biden told Lester Holt of NBC News that Americans should leave Ukraine immediately, following a State Department advisory that “military action may commence at any time and without warning,” as it reissued an advisory urging Americans not to travel to Ukraine with its starkest language yet.
“We’re dealing with one of the largest armies in the world,” Mr. Biden said. “This is a very different situation, and things could go crazy quickly.”
Mr. Sullivan said on Friday that he expected the president to soon meet with Mr. Putin by phone.
The White House would not say if it would engage American troops to evacuate any Americans left in Ukraine should Russia invade. Mr. Sullivan urged Americans who needed assistance to investigate “commercial” options, and to contact the United States embassy.
Earlier Friday, Mr. Biden met by phone with other trans-Atlantic leaders in a call that included Chancellor Olaf Scholz of Germany; Prime Ministers Boris Johnson of Britain, Mario Draghi of Italy and Justin Trudeau of Canada; and Presidents Emmanuel Macron of France, 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 “diplomacy and deterrence,” the White House said.
The call came as Russia builds up its forces around Ukraine — in Belarus, western Russia and Crimea, the territory Russia seized from Ukraine in 2014 — in what U.S. and NATO leaders have said appears to be preparation for an invasion.
Ukraine said on Friday that Russian-backed separatists were holding military exercises in the slice of eastern Ukraine they control, at the same time that Russia holds exercises near Ukraine.