The latest on Ukraine and Russia tensions

White House national security adviser Jake Sullivan encouraged all Americans who remain in Ukraine "to depart immediately" as he detailed how the choreography of a possible Russian invasion of Ukraine could unfold. 

"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," he added.

Sullivan said that if Americans stay they "are assuming risk with no guarantee that there will be any other opportunity to leave and no prospect of a US military evacuation in the event of a Russian invasion."

The national security adviser said that if Russia invades, then it would likely begin with an aerial bombing ahead of a ground invasion. 

"If a Russian attack on Ukraine proceeds, 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," he added.

Sullivan also said: "I am not standing here and saying what is going to happen and not happen; I'm only standing here to say that the risk is now high enough and the threat is immediate enough that prudence demands that it is the time to leave now."

White House national security adviser Jake Sullivan said a potential invasion of Ukraine by Russia "could begin at any time" as the buildup of Russian troops continues.

"As we've said before, we are ready either way. We're ready to continue results oriented diplomacy that addressed the security concerns United States and Russia and Europe consistent with our values and the reciprocity, and we've made that clear to Russia with our allies and partners. We're also ready to respond alongside those allies and partners should Russia choose to take military action," he continued.

His remarks echo Secretary of State Antony Blinken's statement today that an invasion could start at any point.

"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 today.

National security adviser Jake Sullivan is joining today's White House press briefing amid the ongoing situation in Ukraine.

President Biden earlier held a call that lasted more than an hour with global leaders about Russia's buildup of troops near the border between the two countries.

Secretary of State Antony Blinken, meanwhile, said today that a Russian invasion of Ukraine "could begin at any time," including during the Beijing Winter Olympics, and the United States continues "to see very troubling signs of Russian escalation, including new forces arriving at the Ukrainian border."

The top US diplomat made the comments alongside his counterparts from the Quad — Australia, India and Japan — following a day of meetings in Melbourne and after the State Department and Biden warned US citizens to depart Ukraine immediately.

CNN's Jennifer Hansler and Kylie Atwood contributed reporting to this post.

The UK government has advised nationals in Ukraine to leave now while commercial means are still available, the UK Foreign, Commonwealth and Development Office said Friday.

The latest UK travel advice for Ukraine says:

Multiple countries, including Japan, Israel, Norway, South Korea and Latvia, have advised its citizens today to leave Ukraine.

US President Biden has also urged all Americans to leave Ukraine.

The US is 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 and as soon as next week.

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.

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 the State Department for comment.

President Biden's secure video call with transatlantic leaders began at 11:02 a.m. ET and concluded at 12:21 p.m. ET, according to the White House.

CNN reported earlier today that Biden would hold a call with other world leaders this morning on the situation in Ukraine, according to a White House official.

The leaders of Canada, the United Kingdom, France, Germany, Italy, Poland, Romania, NATO, the European Commission, and the European Council were expected to be on the call, according to the official.

CNN's Betsy Klein contributed reporting to this post.  

The US and its allies have new intelligence that suggests Russia could launch an attack on Ukraine even before the end of the Olympics, multiple sources familiar with the matter tell CNN. 

Previous assessments had suggested that Russia was unlikely to move into Ukraine until after the Olympics end on Feb. 20, US officials had told CNN in the past. The revelation of the new intelligence comes as administration officials have dramatically ramped up the urgency of their public warnings related to Ukraine in the past 24 hours.

US Secretary of State Antony Blinken said Friday that a Russian invasion of Ukraine "could begin at any time," including during the Beijing Winter Olympics, and the United States continues "to see very troubling signs of Russian escalation, including new forces arriving at the Ukrainian border."

Kyiv is among the targets identified in the Russian planning, three sources familiar with the new intelligence tell CNN. 

There are ongoing conversations within the administration about declassifying some of that new intelligence, which two US officials said may come later on Friday.

President Biden on Friday morning was set to hold a call with NATO and European allies to discuss the latest intelligence, a White House official told CNN. A European defense official said the North Atlantic Council will be discussing the situation and the new intelligence in a meeting later Friday.  

Biden also joined a meeting of his top national security advisers Thursday evening in the White House Situation Room to discuss the crisis, a person familiar with the meeting said.

When asked about new intelligence, a spokesperson for the Ukrainian defense minister said that such warnings have been heard already.

“Again? These statements have already been heard,” Iryna Zolotar, press secretary for Ukrainian Defense Minister Oleksiy Reznikov, told CNN.

CNN's Alex Marquardt and Mick Krever contributed reporting to this post.

Israel's Ministry of Foreign Affairs will remove the families of Israeli diplomats and employees from its embassy in Kyiv amid growing tensions between Ukraine and Russia.

A ministry statement also urged Israeli citizens in Ukraine to consider leaving the country and recommended those planning to travel to Ukraine to cancel their trip.

The move comes after similar actions today from Japan and South Korea.

US President Biden has also urged all Americans to leave Ukraine.

United Kingdom Defense Secretary Ben Wallace said Friday that he heard "clearly" from the Russian government that they had no intention of invading Ukraine.

Speaking at a news conference in Moscow after talks with Russia's Defense Minister Sergei Shoigu amid Russia-West tensions over Ukraine, Wallace said: "I heard clearly from the Russian government that they had no intention of invading Ukraine."

The UK will look at the actions that back up Russia's assurances that Moscow will not invade Ukraine, Wallace said.

"When they say to me they are not going to invade Ukraine, we will take that seriously, but as I also said we will look at the actions that accompany it," he said.

Wallace also said that "the disposition of the Russian forces that we see over 100,000 in both Belarus and Ukraine, obviously gives that size of force the ability to do a whole range of actions including an invasion of neighboring country at any time."

Wallace said that he had "constructive and frank discussions" with Shoigu, and that they both agreed on the importance of the implementation of the Minsk agreement as "a clear way forward."

"We urged dialogue as a way through to address any concerns that Russia the Russian government may have," Wallace said.

Wallace said he would put the level of UK-Russia relations "above zero" following his meeting with Shoigu Friday, after Russian state news agency RIA-Novosti earlier reported that Shoigu had told Wallace that the level is "close to zero."

"Of course Britain has some ongoing issues with the Russian government, not least, the poisoning and deployment of nerve agent, by agents of the Russian state in Salisbury, and the requests we have, obviously, out for the people suspected of committing that crime. And that is ongoing and needs to be resolved," Wallace said.

However, "this is the first meeting of a UK defense minister with Minister Shoigu since 2013," he said. "So that is nine years, and I think the beginning of this process, which is to understand each other's concerns, also to be able to have a line of communication is a lot better than 0%."

Wallace went on to say that he looks forward to having an ongoing relationship with his Russian counterpart.