President Joe Biden told Russia’s Vladimir Putin that invading Ukraine would cause “widespread human suffering” and that the West was committed to diplomacy to end the crisis but “equally prepared for other scenarios,” the White House said Saturday. It offered no suggestion that the hourlong call diminished the threat of an imminent war in Europe.

Biden also said the United States and its allies would respond “decisively and impose swift and severe costs” if the Kremlin attacked its neighbor, according to the White House.

The two presidents spoke a day after Biden’s national security adviser, Jake Sullivan, warned that U.S. intelligence shows a Russian invasion could begin within days and before the Winter Olympics in Beijing end on Feb. 20.

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Russia denies it intends to invade but has massed well over 100,000 troops near the Ukrainian border and has sent troops to exercises in neighboring Belarus, encircling Ukraine on three sides. U.S. officials say Russia’s buildup of firepower has reached the point where it could invade on short notice.

The conversation came at a critical moment for what has become the biggest security crisis between Russia and the West since the Cold War. U.S. officials believe they have mere days to prevent an invasion and enormous bloodshed in Ukraine. And while the U.S. and its NATO allies have no plans to send troops to Ukraine to fight Russia, an invasion and resulting punishing sanctions could reverberate far beyond the former Soviet republic, affecting energy supplies, global markets and the power balance in Europe.

 “President Biden was clear with President Putin that while the United States remains prepared to engage in diplomacy, in full coordination with our Allies and partners, we are equally prepared for other scenarios,” the White House statement said.

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The call was “professional and substantive” but produced “no fundamental change in the dynamic that has been unfolding now for several weeks,” according to a senior administration official who briefed reporters following the call on condition of anonymity.

The official added that it remains unclear whether Putin has made a final decision to move forward with military action.

Yuri Ushakov, Putin’s top foreign policy aide, said that while tensions have been escalating for months, in recent days “the situation has simply been brought to the point of absurdity.”

He said Biden mentioned the possible sanctions that could be imposed on Russia, but “this issue was not the focus during a fairly long conversation with the Russian leader.”

Before talking to Biden, Putin had a telephone call with French President Emmanuel Macron, who met with him in Moscow earlier in the week to try to resolve the crisis. A Kremlin summary of the call suggested that little progress was made toward cooling down the tensions.

Putin complained in the call that the United States and NATO have not responded satisfactorily to Russian demands that Ukraine be prohibited from joining the military alliance and that NATO pull back forces from Eastern Europe.

In a sign that American officials are getting ready for a worst-case scenario, the United States announced plans to evacuate most of its staff from the embassy in the Ukrainian capital. Britain joined other European nations in urging its citizens to leave Ukraine.

Canada has shuttered its embassy in Kyiv and relocated its diplomatic staff to a temporary office in Lviv, located in the western part of the country, Foreign Affairs Minister Melanie Joly said Saturday. Lviv is home to a Ukrainian military base that has served as the main hub for Canada’s 200-soldier training mission in the former Soviet country.

The timing of any possible Russian military action remained a key question.

The U.S. picked up intelligence that Russia is looking at Wednesday as a target date, according to a U.S. official familiar with the findings. The official, who was not authorized to speak publicly and did so only on condition of anonymity, would not say how definitive the intelligence was.

U.S. Secretary of State Antony Blinken said he told his Russian counterpart Saturday that “further Russian aggression would be met with a resolute, massive and united trans-Atlantic response.”

Meanwhile, Ukrainian President Volodymyr Zelenskyy tried to project calm as he observed military exercises Saturday near Crimea, the peninsula that Russia seized from Ukraine in 2014.

“We are not afraid, we’re without panic, all is under control,” he said.

Ukrainian armed forces chief commander Lt. Gen. Valeriy Zaluzhny and Defense Minister Oleksiy Reznikov issued a more defiant joint statement.

“We are ready to meet the enemy, and not with flowers, but with Stingers, Javelins and NLAWs” — anti-tank and -aircraft weapons, they said. “Welcome to hell!”

U.S. Defense Secretary Lloyd Austin and his Russian counterpart, Sergei Shoigu, also held telephone discussions on Saturday.

Further U.S.-Russia tensions arose on Saturday when the Defense Ministry summoned the U.S. Embassy’s military attache after it said the navy detected an American submarine in Russian waters near the Kuril Islands in the Pacific. The submarine declined orders to leave, but departed after the navy used unspecified “appropriate means,” the ministry said.

Adding to the sense of crisis, the Pentagon ordered an additional 3,000 U.S. troops to Poland to reassure allies.

The U.S. has urged all American citizens in Ukraine to leave the country immediately, and Sullivan said those who remain should not expect the U.S. military to rescue them in the event that air and rail transportation is severed after a Russian invasion.

The Biden administration has been warning for weeks that Russia could invade Ukraine soon, but U.S. officials had previously said the Kremlin would likely wait until after the Winter Games ended so as not to antagonize China.

Sullivan told reporters on Friday that U.S. intelligence shows that Russia could take invade during the Olympics. He said military action could start with missile and air attacks, followed by a ground offensive.

“Russia has all the forces it needs to conduct a major military action,” Sullivan said, adding that “Russia could choose, in very short order, to commence a major military action against Ukraine.” He said the scale of such an invasion could range from a limited incursion to a strike on Kyiv, the capital.

Russia scoffed at the U.S. talk of urgency.

“The hysteria of the White House is more indicative than ever,” said Maria Zakharova, a Russian Foreign Ministry spokeswoman. “The Anglo-Saxons need a war. At any cost. Provocations, misinformation and threats are a favorite method of solving their own problems.”

Zakharova said her country had “optimized” staffing at its own embassy in Kyiv in response to concerns about possible military actions from the Ukrainian side.

In addition to the more than 100,000 ground troops that U.S. officials say Russia has assembled along Ukraine’s eastern and southern borders, the Russians have deployed missile, air, naval and special operations forces, as well as supplies to sustain a war. This week, Russia moved six amphibious assault ships into the Black Sea, augmenting its capability to land marines on the coast.

Biden has bolstered the U.S. military presence in Europe as reassurance to allies on NATO’s eastern flank. The 3,000 additional soldiers ordered to Poland come on top of 1,700 who are on their way there. The U.S. Army also is shifting 1,000 soldiers from Germany to Romania, which like Poland shares a border with Ukraine.

Russia is demanding that the West keep former Soviet countries out of NATO. It also wants NATO to refrain from deploying weapons near its border and to roll back alliance forces from Eastern Europe — demands flatly rejected by the West.

Russia and Ukraine have been locked in a bitter conflict since 2014, when Ukraine’s Kremlin-friendly leader was driven from office by a popular uprising. Moscow responded by annexing the Crimean Peninsula and then backing a separatist insurgency in eastern Ukraine, where fighting has killed over 14,000 people.

A 2015 peace deal brokered by France and Germany helped halt large-scale battles, but regular skirmishes have continued, and efforts to reach a political settlement have stalled.