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.
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 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.
Mr. Putin and Mr. Biden will speak by phone on Saturday, according to the Kremlin and a White House spokesman, and the Kremlin said Mr. Putin would also speak again with President Emmanuel Macron of France. The Kremlin said that the conversation with Mr. Biden would be at the White House’s request; the White House spokesman, speaking on condition of anonymity to describe private conversations, said the Kremlin had suggested a call on Monday, and the White House proposed a call sooner, on Saturday.
Russia’s foreign ministry dismissed American talk of war as mere propaganda.
“A coordinated information attack is being conducted against Moscow,” the ministry said in a statement, along with a list of previous Western warnings of a possible imminent invasion. That messaging, it said, is “aimed at undermining and discrediting Russia’s fair demands for security guarantees, as well as at justifying Western geopolitical aspirations and military absorption of Ukraine’s territory.”
Maria Zakharova, the ministry spokeswoman, wrote on the Telegram app: “The White House’s hysteria is as revealing as ever. The Anglo-Saxons need war at any price.”
Mr. Sullivan said, “Russia could choose in very short order to commence a major military action against Ukraine,” but added that officials could not be sure exactly when, or if, Mr. Putin may decide to invade. As he spoke, Mr. Biden 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 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 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 “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.
