President Biden warned President Vladimir V. Putin of Russia in a one-hour phone call Saturday that invading Ukraine would lead to “swift and severe” costs.
The call came amid heightened tensions, and just hours after the State Department ordered all but a “core team” of its diplomats and employees to leave the American Embassy in Kyiv over fears that Moscow would soon mount a major assault.
According to a White House statement, Mr. Biden was clear in the phone call that while the United States remained prepared to engage in diplomacy, in coordination with allies, it was also prepared for other scenarios.
The Russian government was expected to present its take on the call soon.
Reflecting the urgent concern in Washington over Russia’s growing military buildup surrounding its smaller neighbor, the Pentagon said it would temporarily pull 160 American military trainers out of the country, where they had been working with Ukrainian troops near the Polish border.
Even as Mr. Biden and Mr. Putin spoke by telephone — and after calls earlier Saturday between the top U.S. and Russian diplomats and between the countries’ defense secretaries — the path to a diplomatic resolution to the standoff appeared to be narrowing, with growing numbers of Russian and Russian-backed forces massing around Ukraine on three sides.
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. But 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 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 Ukrainian government urged calm, with President Volodymyr Zelensky saying that he had not seen intelligence indicating an imminent Russian attack, and that “too much information” about a possible offensive was sowing unnecessary fear.
The United States has ruled out sending troops to defend Ukraine, but it has increased deployments to NATO member countries in Eastern Europe. The Pentagon on Friday said it had ordered 3,000 more soldiers to Poland.
The White House is eager to avoid a repeat of the chaotic evacuation of the U.S. Embassy staff from Kabul last August as Afghanistan fell to the Taliban. The United States and countries including Britain, Denmark, Germany, Japan, Latvia and the Netherlands have issued increasingly urgent calls for their citizens to leave Ukraine. On Saturday, KLM, the main Dutch airline, announced that it will stop flying to Ukraine, citing the security situation.
A State Department official emphasized on Saturday that the U.S. military would not be evacuating American citizens from Ukraine in the way troops did in Afghanistan.
Russia has accused Western countries of spreading misinformation about its intentions. On Saturday, its Foreign Ministry said it was pulling some of its diplomatic personnel out of Ukraine because it was “drawing the conclusion that our American and British colleagues seem to know about certain military actions.”