There is a "very distinct possibility" that Russia will invade Ukraine in a "reasonably swift time frame," White House national security adviser Jake Sullivan warned Friday, urging all Americans in Ukraine to "leave as soon as possible, and in any event within the next 24 to 48 hours."

"If you stay, you are assuming risk with no guarantee that there will be any other opportunity to leave, and no prospect of a U.S. military evacuation in the event of a Russian invasion," Sullivan said in the White House briefing room. An attack, he said, would be "likely to begin" with aerial bombing and missile strikes, and "no one would be able to count on air or rail or road departures."

As U.S. warnings sharply escalated, the White House said President Joe Biden would speak with Russian President Vladimir Putin late Saturday morning, Washington time. The Kremlin said Biden had requested the call. U.S. officials said the Russians proposed it take place Monday but accepted Biden's Saturday counterproposal.

The announcement of the call came amid reports of new intelligence - and evidence on the ground - over the past several days indicating that Russia, with 130,000 troops and major weaponry surrounding Ukraine on three sides, is now fully prepared to launch an invasion. Sullivan said the United States has no confirmation that Putin has made the final decision to attack. But, he said, "we believe he very well may give the final go order. That is a very distinct possibility."

"It may well happen," he said. "It may well happen soon."

As diplomatic efforts to resolve the Ukraine crisis remained at a stalemate, Russia kicked off a second day of major military exercises near Ukraine's borders in Belarus that analysts say could presage an invasion.

Biden held a call early Friday with his counterparts in Canada, Britain and other NATO nations to further coordinate "diplomacy and deterrence," the White House said.

As the mood also darkened in Europe, ambassadors from NATO member countries convened in Brussels to discuss the continued Russian buildup, a U.S. official said.

U.S. Secretary of State Antony Blinken said during a visit to Australia that the West continues to see "troubling signs of Russian escalation, including new forces arriving at the Ukrainian border."

"As we've said before, we're in a window when an invasion could begin at any time, and to be clear, that includes during the Olympics," Blinken said. He referred to speculation among some officials that Putin may wait for the conclusion of the Winter Games in Beijing to avoid angering China, its key partner. The military exercises in Belarus are scheduled to end on Feb. 20, the day the Olympics conclude.

According to one Western official, Moscow's military preparations for a full-scale invasion are complete. "Militarily it can start within days," the official said, speaking on the condition of anonymity because of the matter's sensitivity. "It's up to Putin."

The U.S. judgment that Putin is likely to launch an attack, potentially as early as next week, is based in part on new intelligence that Russia is planning to conduct a false-flag operation to create a pretext for invading Ukraine, according to multiple officials who spoke on the condition of anonymity to discuss administration deliberations.

The precise date and nature of the alleged Russian operation was unclear. U.S. officials had earlier accused Russia of planning to stage and film a fake attack by Ukrainian military forces on Russia as a pretext for invasion.

On Thursday evening, a meeting in the White House Situation Room was quickly convened to discuss the latest developments.

Sullivan declined to discuss specific intelligence assessments, but he said that "the intelligence community has sufficient confidence that I can stand before you today and say ... there is a distinct possibility that Vladimir Putin would order a military action, an invasion of Ukraine, in this window," potentially including "the time period before February 20th."

The intelligence community, he added, believes that "everything I have just said is well-grounded, in both what they are seeing on the ground and what they are picking up through all of their various sources."

Reminded of faulty intelligence that preceded the U.S. invasion of Iraq, Sullivan said there was a "fundamental distinction" between the two situations. "In Iraq, intelligence was used and deployed ... to start a war," he said. "We are trying to stop a war."

"All we can do is come here before you in good faith and share everything that we know ... while protecting [intelligence] sources and methods," Sullivan said.

"Today, we are talking about more than 100,000 Russian troops amassed along the Ukrainian border," he said, noting that photos of the deployments were "all over social media."

A senior European diplomat confirmed that the United States had shared intelligence leading to a new sense of urgency and noted that a number of countries are now telling their citizens to evacuate immediately. Britain's Foreign Office issued an advisory that "British nationals in Ukraine should leave now while commercial means are still available." Denmark and Norway issued similar advice.

Sullivan urged the thousands of U.S. citizens believed to be in Ukraine to leave immediately, saying, "The risk is now high enough and the threat is now immediate enough that this is what prudence demands." He said that, unlike in Afghanistan, there would be no U.S. military evacuation.

"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," he said. "So we are asking people to make the responsible choice."

U.S. officials confirmed Friday that an additional 3,000 troops from the 82nd Airborne Division would be sent to Poland, adding to the 1,700 troops already dispatched to that country. The new deployment was first reported by Reuters.

Gen. Mark Milley, chairman of the Joint Chiefs of Staff, spoke Friday by telephone with his Russian counterpart, Gen. Valery Gerasimov. The two "discussed several security-related issues of concern" but as with previous calls between them "agreed to keep the specific details of their conversation private," a Milley spokesperson said. Milley also spoke with counterparts from Canada, France, Germany, Italy, Poland, Romania and the United Kingdom, as well as the head of the NATO military committee on "adjustment of the U.S. force posture in Europe."

Sullivan emphasized that U.S. troops are not "being sent to go fight Russia in Ukraine. They are not going to war in Ukraine. They are not going to war with Russia." U.S. forces, he said, are in place in NATO countries to reinforce and defend the alliance within its own territory.

Ukraine is not a member of NATO. To defend it, the United States and other allies have sent hundreds of millions of dollars of defensive military equipment and trained its armed forces. The allies have promised swift and severe economic sanctions, designed to cripple a significant portion of the Russian economy and industry, if Moscow launches military action against Ukraine. They have offered to discuss and address Russia's security concerns about NATO expansion in Eastern Europe but not to meet Putin's principal demand that the alliance promise never to admit Ukraine.

In Ukraine, Foreign Minister Dmytro Kuleba invoked diplomatic agreements to ask Russia for detailed information about its exercises, including precise locations, numbers of troops and dates of completed activity. Russia has 48 hours to comply, Kuleba said. Noncompliance could escalate to emergency meetings within the Organization for Security and Cooperation in Europe, experts said.

Horton reported from Kyiv, Ukraine, and Cheng from Seoul. The Washington Post's Robyn Dixon in Moscow, Karla Adam in London and Ellen Nakashima, Ashley Parker and Dan Lamothe in Washington contributed to this report.