President Biden plans to speak with President Vladimir V. Putin of Russia on Saturday in a bid to defuse the crisis over Ukraine, with Washington warning that Moscow could mount a major assault on Ukraine at any time.
Secretary of State Antony J. Blinken also said that he would speak with Sergey V. Lavrov, Russia’s foreign minister, on Saturday to press for Russia to pull back the land, sea and air forces it has built up on three sides of Ukraine, and to engage in diplomacy to resolve what has grown into one of the gravest security threats in Europe since the Cold War.
“We’re in the window when a Russian invasion can start at any time if President Putin so decides,” Mr. Blinken told reporters on Saturday in Fiji, where he was on a weeklong tour of the Pacific.
U.S. intelligence officials had 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, adding that an invasion could begin “during the Olympics,” which are scheduled to end on Feb. 20.
U.S. officials 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.
Mr. Blinken said that U.S. officials continued 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.
The Kremlin said that the conversation with Mr. Biden would be at the White House’s request; the White House spokesman, speaking on the condition of anonymity to describe private conversations, said the Kremlin had suggested a call on Monday and that the White House proposed a call sooner, on Saturday. The Kremlin said Mr. Putin would also speak again on Saturday with President Emmanuel Macron of France.