WASHINGTON — The United States warned Friday that President Vladimir Putin of Russia could mount a major military assault on Ukraine at any moment, suggesting a crisis that had been building for months has reached a critical phase.

The Pentagon, which has ruled out deploying troops to defend Ukraine, sent 3,000 soldiers to neighboring Poland Friday as tensions mounted, reinforcing the U.S. military personnel being dispatched to help NATO allies. A host of countries, fearing an imminent invasion, told their citizens to leave Ukraine. And President Joe Biden spent more than an hour on a call with allies to discuss “diplomacy and deterrence,” the White House said.

Ukraine warned that drills by Russia and Russian-backed separatists had left the country all but encircled and its ports effectively blockaded, the latest evidence of a shift in tone after weeks in which Ukraine’s leaders had downplayed the threat of an attack.

With the United States pushing for a diplomatic solution, Putin and Biden will speak by phone Saturday, according to the Kremlin and a White House spokesperson, and the Kremlin said Putin would also speak again with President Emmanuel Macron of France.

U.S. intelligence officials had initially thought Putin would wait until the end of the Winter Olympics in Beijing later this month before deciding whether to go ahead with an offensive, to avoid antagonizing President Xi Jinping of China, a critical ally. In recent days, however, new intelligence and further Russian troop deployments prompted a change in their assessment. U.S. officials said it was unclear whether Putin had made a decision to invade.

U.S. officials have picked up intelligence that Russia is considering Wednesday as the possible date for the start of military action, according to multiple officials briefed on the material. Those officials, who spoke on condition of anonymity to discuss sensitive information, acknowledged the possibility that the mention of a particular date could be part of a Russian disinformation effort.

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The combination of the Russian troop movements and the new intelligence prompted a cascade of public warnings by the United States and other countries — including the Netherlands, Latvia, Britain, Japan and even Russia — to any citizens remaining in Ukraine. Many others began evacuating embassy staff.

“The Russians are in a position to be able to mount a major military action in Ukraine any day now,” Jake Sullivan, Biden’s national security adviser, told reporters at the White House Friday, adding that an invasion could begin during the Olympics, which are scheduled to end Feb. 20.

“We don’t know exactly what is going to happen,” Sullivan said, emphasizing the need for Americans to leave Ukraine now. “The risk is now high enough, and the threat is now immediate enough that this is what prudence demands. If you stay, you are assuming risk with no guarantee that there will be any opportunity to leave and no prospect of the U.S. military evacuation in the event of a Russian invasion.”

Russia has made a series of demands of the West, including scaling back the NATO military presence in Eastern Europe to 1990s levels and guaranteeing that Ukraine could never join NATO. (Putin has long been vehemently opposed to Ukraine, a former pillar of the Soviet Union, joining NATO, a position he last made forcefully clear when Russian forces reclaimed Crimea in 2014.) The United States has called those demands “non-starters’’ and instead offered a series of proposals aimed at arms control.

“What I do know about Putin is he likes uncertainty,” said Michael McFaul, a former U.S. ambassador to Russia. “He has leveraged that in the past for advantage. He is forcing Biden’s hand and everybody else’s.”

Next week, German Chancellor Olaf Scholz is scheduled to visit Ukraine’s capital, Kyiv, and Moscow, fresh from a visit to Washington where he and Biden promised a “united” front on shutting down Nord Stream 2, a lucrative Germany-to-Russia gas pipeline project, should Russia invade Ukraine.

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Russia’s foreign ministry dismissed U.S. 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 spokesperson, wrote on the Telegram app: “The White House’s hysteria is as revealing as ever. The Anglo-Saxons need war. At any price.”

Sullivan disagreed with the idea that informing Americans of Russia’s military capabilities was the same as calling for a war. “We are trying to stop a war. Prevent war. To avert a war,” he told reporters.

U.S. officials have warned of a grim toll if 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. Sullivan said Friday that officials believed that an attack would likely start with missile and aerial attacks, and continue with a ground invasion.

On Friday, Biden spoke with other trans-Atlantic leaders on a call that included Scholz; 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 NATO Secretary-General Jens Stoltenberg.

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The leaders met for about 80 minutes, in a call that was initially supposed to be centered on “diplomacy and deterrence,” the White House said.

Gen. Mark Milley, chair of the Joint Chiefs of Staff, also spoke Friday with his Russian counterpart as well as allied chiefs of defense about the crisis in Ukraine, military officials said.

The calls came as Russia builds up its forces in Belarus, western Russia and Crimea. Ukraine said Friday that Russian-backed separatists were holding military exercises in the slice of eastern Ukraine they controlled, at the same time that Russia holds exercises near Ukraine.

The drills tested the separatists’ preparation for live-fire operations, practicing “driving artillery, tanks and armored vehicles” in field exercises, the Ukrainian statement said. Some units of the force, believed to number 30,000 troops, were put on their highest level of alert, the Ukrainians said, and senior Russian military officers were observing the activity.

The assessment from Kyiv was the latest evidence of a shift by officials there to more alarming commentary about the military risk facing the country. That follows weeks of efforts to minimize the threat of an invasion, seeking to calm the public, limit the economic fallout and avoid anything that could be deemed a provocation by Moscow.

“If there’s a war tomorrow, Putin has calculated that Zelenskyy will be blamed for not preparing for war,” McFaul said, referencing President Volodymyr Zelenskyy of Ukraine. “That doesn’t get a lot of attention in our press, but that’s a very big part of his strategy. Ideally, he would like to see democracy in Ukraine fall, and Zelenskyy personally fall.”

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Ukraine this week began its own nationwide military exercises to coincide with joint Russian and Belarusian exercises to the north of Ukraine, in Belarus, only 140 miles from Kyiv.

Those joint exercises involved a flurry of military activity Friday, the Russian Ministry of Defense said in a statement. Troops practiced evacuating the wounded from the battlefield, maneuvering with armored vehicles and reconnaissance activities. Russia’s air force jets fired at an airborne target.

To the south, the Russian navy announced Thursday the closure of large swaths of the Black Sea for live-fire exercises by its fleet that will effectively blockade Ukrainian ports, including the port of Odessa. The naval exercises were scheduled to begin Sunday and last six days.

Russia has massed armored vehicles and soldiers near its borders to the northeast of Ukraine and in the south on the Crimean Peninsula, as well as in Belarus.

A meeting in Moscow Friday between the British defense secretary, Ben Wallace, and his Russian counterpart, Sergei Shoigu, was cordial but led to a dim assessment of relations between Russia and the West by Shoigu.

After Wallace laid a wreath at the Tomb of the Unknown Soldier in Moscow, acknowledging Russia’s loses in World War II, Shoigu nodded to the countries’ alliance in that war but added, “Unfortunately, the level of our cooperation is close to zero and is about to cross the zero meridian and reach the negatives.”

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In news conferences that extended into early Friday morning in Berlin, both Russian and Ukrainian negotiators said a channel of talks supported by Macron had brought no breakthroughs.

“It would be good if during the second meeting we could agree on something,” said Andriy Yermak, Ukraine’s chief negotiator. After nine hours of talks, the negotiators could not agree on a joint statement.

Separate talks in Moscow Thursday between the British foreign secretary, Liz Truss, and Russia’s foreign minister, Sergey Lavrov, also went nowhere, with Lavrov comparing them to “the conversation of a mute person with a deaf person.”