(WASHINGTON) — The U.S. issued a stark new warning Friday that a Russian invasion of Ukraine could begin during the Olympics.

“I do want to be clear: it could begin during the Olympics despite a lot of speculation that would only happen after the Olympics,” scheduled to end Feb. 20, national security adviser Jake Sullivan told reporters at the White House, but he quickly added that the U.S. could not say whether Russian President Vladimir Putin had made a decision to do so.

Sullivan also said the situation had grown so dire that Americans in Ukraine should leave “immediately” — within the next 24 to 48 hours.

“We don’t know exactly what is going to happen. But the risk is now high enough, and the threat is now immediate enough that this is what prudence demands,” he said.

“If you stay you are assuming risk with no guarantee that there will be any other opportunity to leave, and there is no prospect of a U.S. military evacuation in the event of a Russian invasion,” he added, echoing what President Joe Biden said in an NBC News interview Thursday.

Sullivan said the U.S. is reducing the size of its “embassy footprint” in Kyiv.

Pressed by reporters about the evidence the U.S. had, Sullivan there is a “credible prospect Russian military action will happen even before the end of the Olympics.”

He went on to describe in vivid detail what could happen, including a “rapid assault on the city of Kyiv.” He said Biden wouldn’t put U.S. service members’ lives at risk in a war zone to rescue people who don’t leave now.

“If a Russian attack on Ukraine precedes it is likely to begin with aerial bombing and missile attacks that could obviously kill civilians without regard to their nationality. A subsequent ground invasion would involve the onslaught of a massive force with virtually no notice, communications to arrange a departure could be severed and commercial transit halted,” Sullivan said.

“The president will not be putting the lives of our men and women in uniform at risk by sending them into a war zone to rescue people who could have left now but chose not to. So, we’re asking people to make the responsible choice,” he said.

Earlier Friday, Biden held a call with transatlantic leaders to chart next moves as talks over Russia’s military build-up near Ukraine showed no sign of defusing the crisis.

Biden spoke about “coordination on both diplomacy and deterrence” with the leaders of Canada, France, Germany, Italy, Poland, Romania, the United Kingdom, NATO, the European Commission, and the European Council, according to the White House.

The president has remained largely silent on Ukraine over the past few days, instead holding public events focused on the U.S. economy.

The transatlantic call came as NATO warned Europe was facing a “dangerous moment.”

“This is a dangerous moment for European security,” NATO Secretary-General Jens Stoltenberg said Friday in Brussels.

European leaders have engaged in intense diplomacy with Russia and Ukraine over the past several weeks to avoid war in eastern Europe. But the talks have so far failed to yield much apparent progress.

France’s President Emmanuel Macron traveled to Moscow to meet with his Russian counterpart, Vladimir Putin, before meeting with Ukraine’s President Volodymyr Zelenskyy in the Ukrainian capital, Kyiv, the next day.

Russia and Ukraine held talks Thursday in Berlin, moderated by Germany and France, but after nine hours of discussion failed to even agree on issuing a joint statement.

Western officials had hoped that the latest round of the so-called “Normandy Format Talks” would push forward the diplomacy by Macron and other officials who have been shuttling between capitals over the past couple weeks.

The sides remained at an impasse, though, over Russia’s insistence that the Ukrainian government speak directly with Russian-backed separatist leaders in eastern Ukraine.

Biden said Monday that Americans currently in Ukraine should leave, and on Thursday, he repeated that message with more urgency.

“American citizens should leave now,” Biden Thursday said in an interview with NBC News. “It’s not like we’re dealing with a terrorist organization. We’re dealing with one of the largest armies in the world. It’s a very different situation and things could go crazy quickly.”

Senior U.S. officials say they do not believe Russia’s President Vladimir Putin has made a decision whether to invade Ukraine, even as he has amassed over 100,000 troops on Russia’s border with Ukraine.

The U.S. and other Western nations have warned of severe economic consequences to Russia if it does invade. Russia denies it plans to do so.

Meanwhile, on Thursday, Russia and Belarus kicked off 10 days of joint exercises in Belarus, north of Ukraine.

“As we said before, we’re in a window when an invasion could begin at any time,” Secretary of State Antony Blinken said Friday in Melbourne, Australia. “To be clear, that includes during the Olympics.”

The Winter Olympics, which are ongoing in Beijing, are scheduled to end on Feb. 20.

ABC News’ Patrick Reevell contributed to this report.

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