President Joe Biden plans to speak with Russian President Vladimir Putin on Saturday, a person familiar with the matter said, as the US warns Russia could attack Ukraine using bombs and missiles at any moment. The high-stakes talks come at what US officials describe as a critical juncture in the ongoing crisis. A significant increase of Russian ground forces and military assets have surrounded Ukraine, and Putin could decide at any moment to activate them into a deadly invasion. He hasn’t decided whether to act, the White House said Friday. But that has not stopped American officials from dramatically increasing their warnings an attack is now a “distinct possibility” and could occur swiftly. Biden’s phone conversation with Putin – scheduled for 11 a.m. ET Saturday, according to the Kremlin – will be his first since the end of December. Since then, the number of Russian troops near Ukraine has increased and the prospects of an invasion have increased, according to American intelligence assessments. Putin has also engaged a series of Western leaders in talks that have so far appeared fruitless in defusing the situation. The Russian Ministry of Foreign Affairs Friday accused Western countries and the media is spreading a “large-scale disinformation campaign,” which promotes the thesis about an allegedly impending Russian invasion of Ukraine. “At the end of 2021 and the beginning of 2022, the global information space faced a media campaign unprecedented in its scale and sophistication, the purpose of which is to convince the world community that the Russian Federation is preparing an invasion of the territory of Ukraine,” the Ministry said in a statement published on its website, accusing Western nations and media outlets of spreading disinformation “in order to divert attention from their own aggressive actions.” US national security adviser Jake Sullivan said Friday the two men would speak by phone but didn’t specify when. The President was planning the spend the weekend at the Camp David presidential retreat. Earlier Friday, Sullivan warned a Russian assault on Ukraine could begin soon, beginning with aerial bombings and missile attacks. He advised all Americans to depart the country for their own safety as quickly as possible. “Any American in Ukraine should leave as soon as possible, and in any event in the next 24 to 48 hours,” Sullivan said. “We obviously cannot predict the future, 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.” “If a Russian attack on Ukraine proceeds, it is likely to begin with aerial bombings and missile attacks that could obviously kill civilians without regard to their nationality. A subsequent ground invasion would obviously involve the onslaught of a massive force,” he continued. The warning came just before a senior defense official told CNN that Defense Secretary Lloyd Austin had ordered 3,000 more soldiers from the 82nd Airborne Division to deploy to Poland, joining the 1,700 already there. The soldiers are there to help Americans who may try to leave Ukraine. The State Department is calling Americans in Ukraine to find out if they have plans to leave as the US continues to urge Americans to leave the country. Lee Humerian, an American living in Ukraine with his family and working as a missionary, told CNN that he got a call from the State Department Friday, asking if he had plans to leave the country. The official asked if he read the most recent email from the State Department encouraging Americans to leave, according to Humerian. He told the official he had read it and he did not have plans to leave. Humerian registered with the State Department’s program that tracks Americans abroad. The State Department did not immediately respond to request for comment about these calls. Despite those warnings, Sullivan added that it’s not clear if Putin has decided to invade Ukraine. “I want to be crystal clear. A final decision has not been taken by President Putin, but we have a sufficient level of concern based on what we’re see on the ground and what our intelligence analysts have picked up that we’re sending this clear message and it remains a message that we have now been sending for some time. And it is an urgent message because we’re in an urgent situation,” Sullivan said. Later, he said there was a “very distinct possibility” that Russia would act militarily, but couldn’t pinpoint when or how. “I’m not going to get into intelligence information, but if you look at forces in both Belarus and in Russia on the other side of the Ukrainian border from the north and from the east, the Russians are in a position to be able to mount a major military action in Ukraine in day now,” he said. Earlier Friday, US Secretary of State Antony Blinken said the US believes a Russian invasion of Ukraine could begin “at any time,” including during the ongoing Olympics in Beijing. He also said the US continues “to see very troubling signs of Russian escalation, including new forces arriving at the Ukrainian border.” Abrupt Situation Room meeting On Thursday evening, Biden’s aides attended an abruptly scheduled meeting in the basement Situation Room that was called in the context of Russia’s escalating military buildup. It had not been organized in advance, a source familiar with the meeting said. Biden joined for part of the meeting. Afterward, officials began warning publicly that Russia could invade Ukraine before the end of the Beijing Olympics, which conclude February 20. “We continue to see signs of Russian escalation including new forces arriving at the Ukrainian border. As we’ve said before, we are in the window when an invasion could begin at any time should Vladimir Putin decide to order it,” Sullivan said Thursday. “I do want to be clear, it could begin during the Olympics, despite a lot of speculation that it will only happen after the Olympics.” “Russians are in a position to be able to mount a major military action in Ukraine any day now,” Sullivan said. The urgent messages from Biden’s top aides came a week after the administration had decided to no longer use the word “imminent” to describe the likelihood of an invasion. “I used that once. I think others have used that once. And then we stopped using it because I think it sent a message that we weren’t intending to send, which was that we knew President Putin had made a decision,” press secretary Jen Psaki said. US planning to pull observers from Ukraine, send troops to Poland The fears over an invasion has led to the US planning to pull its impartial observers from the Organization for Security and Cooperation in Europe mission in Ukraine as the security environment deteriorates, two sources familiar with the discussions said. The Biden administration is also discussing the possibility of further reducing the number of staff at the US Embassy in Kyiv and pulling other US government employees out of Ukraine amid new US intelligence suggesting that Russia could move to attack Ukraine prior to the end of the Beijing Olympics next week. The US has estimated that Russia has more than 100,000 troops near the Ukraine border, with thousands added just this week, according to an administration official. “We’re continuing to draw down our embassy,” Blinken said during a press conference on Friday. “We will continue that process. And we’ve also been very clear that any American citizens who remain in Ukraine should leave now.” OSCE observers’ “main tasks are to observe and report in an impartial and objective way on the situation in Ukraine; and to facilitate dialogue among all parties to the crisis,” according to their website. They also have been responsible for tracking ceasefire violations in eastern Ukraine. The US last month ordered the departure of diplomats’ family members from the US Embassy in Kyiv and allowed nonessential personnel to leave on a voluntary basis. CNN has reached out to OSCE and State Department for comment. As the plans are made for the observers are made to leave Ukraine, the troops from the 82nd Airborne will be heading to neighboring Poland. The soldiers will fall under the command of Maj. Gen. Christopher Donahue, who was the commander of forces during the final evacuation and withdrawal from Afghanistan. Three hundred members of the 18th Airborne Corps have also arrived in Germany, the official said. They are under the command of Lt. Gen. Michael Kurilla, who’s slated to be the next head of US Central Command. This group of 5,000 troops will “reassure our NATO allies, deter any potential aggression against NATO’s eastern flank, train with host-nation forces, and contribute to a wide range of contingencies,” the senior defense official said. On Wednesday, CNN reported that the White House has approved a plan for members of the 82nd Airborne in Poland to help Americans who may try to leave Ukraine. The troops will begin setting up processing areas and temporary shelters. ‘The risk of conflict in Europe is real’ Biden on Thursday urged Americans in Ukraine to leave the country immediately, warning that “things could go crazy quickly” in the region. “American citizens should leave now,” Biden said in an interview that was taped Thursday with NBC News’ Lester Holt. The President spoke with the leaders from Canada, the United Kingdom, France, Germany, Italy, Poland, Romania, NATO, the European Commission, and the European Council in a call that lasted more than an hour Friday morning. Some leaders on the call later expressed concerns about the risk of invasion and urged the redoubling of diplomatic efforts with Russia. NATO Secretary General Jens Stoltenberg said in a statement that “the risk of conflict in Europe is real.” “Participants made clear that any further Russian aggression against Ukraine would come at a high cost, while reiterating that they were ready to continue dialogue with Russia,” he said. A Downing Street spokesperson said United Kingdom Prime Minister Boris Johnson had told allies on the call that he feared for the security of Europe due to the situation in Ukraine. “He impressed the need for NATO allies to make it absolutely clear that there will be a heavy package of economic sanctions ready to go, should Russia make the devastating and destructive decision to invade Ukraine,” the spokesperson said. Later Friday, a British diplomat told CNN that staff are not evacuating the British Embassy in Kyiv, but they are “temporarily removing all nonessential staff and dependents” and “a core team will remain to continue with essential duties.” The French government said in a statement that the leaders on the call with allies discussed Russian military exercises that recently began in Belarus, stating that the “leaders remain extremely vigilant in this regard.” French President Emmanuel Macron and Putin will speak on Saturday, according to Elysee Palace. Tweets from the German government about the meeting said “(a)llies are determined to impose swift sanctions on Russia, should there be further violations of Ukraine’s territorial integrity and sovereignty” and that “(t)he goal is to prevent a war in Europe.” And the European Commission said the group had underscored its support for Ukraine and its final adoption of an emergency financial assistance package. European Commission President Ursula Von der Leyen indicated that sanctions against Russia would concern the financial and energy sectors, as well as exports of high-tech products, according to a statement. This story has been updated with more reporting.