A satellite image shows deployment of a troop housing area and milvitary vehicles in Rechitsa, Belarus, this week.Credit...Maxar Technologies., via Reuters

WASHINGTON — The Biden administration warned on Friday that President Vladimir V. Putin of Russia could mount a major assault on Ukraine at any time, having built up formidable land, sea and air forces on three sides of its smaller neighbor.

U.S. intelligence officials had initially thought Mr. Putin was prepared to wait until the end of the Winter Olympics in Beijing before possibly ordering an offensive, 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.

“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, adding that an invasion could begin “during the Olympics,” which are scheduled to end on Feb. 20, and warned that all Americans should leave Ukraine in the next 24 to 48 hours.

U.S. officials still do not know whether Mr. Putin has decided to invade, Mr. Sullivan insisted. “We are ready either way,” he said. “Whatever happens next, the West is more united than it has been in years.”

The United States has picked up intelligence that Russia is discussing next Wednesday as the target date for the start of military action, officials said, acknowledging the possibility that mentioning a particular date could be part of a Russian disinformation effort. The combination of the Russian troop movements and the new information about a possible date helped to trigger the flurry of diplomatic activity and public warnings by the NATO allies on Friday.

Antony J. Blinken, the secretary of state, told reporters at a news conference in Melbourne that an “invasion could begin at any time. And, to be clear, that includes during the Olympics.” Mr. Blinken added that U.S. officials “continue to see very troubling signs of Russian escalation, including new forces arriving at the Ukrainian border.”

The United States and many other countries — including, recently, Britain, Japan, Norway, Latvia and Denmark — have issued a series of increasingly urgent calls for their citizens to leave Ukraine. The U.S. has ruled out sending troops to defend Ukraine, but has increased deployments to NATO member countries in Eastern Europe, and on Friday the Pentagon ordered 3,000 more soldiers to Poland.

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. As Mr. Sullivan spoke, the president was preparing to depart for Camp David for the weekend — the whir of Marine One’s blades could be heard in the White House briefing room.

“The risk is now high enough and the threat is now immediate enough that this is what prudence demands,” Mr. Sullivan 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.”

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,” its starkest language yet. And the president said that in the event of all-out war, the United States would not send troops to rescue Americans and risk a clash with Russia.

“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.

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.

— Katie Rogers

Sources: nytimes.com