The latest on Ukraine and Russia tensions

US President 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 upcoming interview that was taped Thursday with NBC News' Lester Holt.

Biden said during his NBC interview that "there's not" a situation that could prompt him to send US troops to rescue Americans attempting to exit Ukraine, adding, "That's a world war when Americans and Russia start shooting at one another."

If Russian President Vladimir Putin is "foolish enough to go in, he's smart enough not to ... do anything that would negatively impact on American citizens," Biden added.

The White House has approved a plan for the nearly 2,000 US troops in Poland to help Americans who may try to evacuate Ukraine if Russia invades, according to two US officials familiar with the matter.

The US forces are not currently authorized to enter Ukraine itself if a war breaks out, and there are no plans for them to conduct a noncombatant evacuation operation akin to the US operation in Afghanistan last summer. Instead, the plan as it now stands is that the troops, who are from the 82nd Airborne Division, will begin setting up processing areas and temporary shelters inside Poland near Ukraine's border where Americans fleeing the country could go for help while in transit. The facilities have not yet been stood up, one defense official said, but will start to be as more US troops arrive in Poland.

The advisory told American citizens in Ukraine to "be aware that the US government will not be able to evacuate US citizens in the event of Russian military action anywhere in Ukraine."

In late January, the State Department authorized the departure of non-emergency personnel from the US Embassy in Kyiv and ordered family members to depart the country.

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.

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The North Atlantic Treaty Organization is looking into a longer-term presence in eastern Europe as it continues to reinforce its flank in the region, Secretary-General Jens Stoltenberg said Friday.

“We are also assessing whether we should just have a more longer-term presence in eastern border lines,” he said in a joint press conference with Romanian President Klaus Iohannis.

“That includes a battle group in Romania but also perhaps battleships in all the parts of the southeast of the Alliance," he added.

NATO's eastward expansion is a core concern of Russia, which has reaffirmed this during recent diplomatic efforts from the West to prevent an invasion of Ukraine.

A longer-term presence "will be discussed and addressed at the upcoming NATO defense ministerial meeting next week," Stoltenberg said. 

"I expect that ministers will agree to further start the planning and to address the scale and the scope and the details about how to deploy a battlegroup, what kind of battlegroup and then I expect that the final decision will be taken during the spring,” he said.

The pair spoke at Romanian military air base Mihail Kogălniceanu, where another 1,000 US troops were set to arrive Friday, bringing the total number of US service members to almost 2,000 at the base, Stoltenberg said. 

Iohannis affirmed the importance of defending NATO’s eastern flank under threat from Russia which continues to amass troops along its border with Ukraine, which he described as "one of the worst crises after the fall of the Iron Curtain."

NATO is ready to deploy more troops and equipment on short notice if needed, Stoltenberg said, while reaffirming the alliance is solely defensive.

Talks held in Berlin on Thursday between advisers from France, Germany, Russia and Ukraine regarding tensions around Ukraine were "long and difficult," an Élysée spokesperson told journalists Friday. 

The German chancellor's spokesperson Steffen Hebestreit also said the talks were "difficult," adding they are seen as "an intermediate step."

“Russia agreed to the negotiation in principle, but ultimately insisted that it be written that Ukraine is to negotiate directly with the separatists, which is Ukraine's only red line,” the French spokesperson said. 

Hebestreit said that "it is good that talks are continuing — that what is the basis of the Minsk agreement continues to be accepted by all parties." 

Hebestreit said at a press briefing that the so-called Normandy talks will resume in March. 

Both Japan and South Korea told their citizens to evacuate Ukraine immediately amid the Russian buildup of troops on the border.

Japan's Foreign Ministry told residents that "the situation continues to be unpredictable."

"While there have been diplomatic efforts by the countries involved, there is a growing possibility that the situation could deteriorate rapidly. For this reason, the entire country of Ukraine has been raised to Level 4," the ministry said.

The South Korean government raised its travel alert level for Ukraine to 4, the highest level, banning travel to Ukraine and ordering its nationals in the country to evacuate, according to a news release from the South Korean Ministry of Foreign Affairs Friday.

The ministry said the level 4 alert comes into effect on Saturday 5 p.m. Ukraine time.

The ministry said this is a preventive measure in case the local situation rapidly worsens in Ukraine.

As of Friday, there are a total of 341 South Korean nationals in Ukraine, the release said.

South Korea previously advised its nationals residing in Kyiv and 14 other Ukrainian oblasts to leave amid heightened tensions between Ukraine and Russia on Jan. 25.

US President Biden is holding a call with other world leaders on the situation in Ukraine this morning, a White House official told CNN.

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

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."

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 US President Biden warned US citizens to depart Ukraine immediately.

Prior to the start of the Olympics earlier this month, Deputy Secretary of State Wendy Sherman suggested that the Winter Games, which are hosted by China, could impact Russian President Vladimir Putin's thinking about the timing of a potential invasion. Putin traveled to Beijing for the beginning of the Olympics and met with Chinese President Xi Jinping. The two released a lengthy pact pledging no limits to their cooperation.

Blinken told reporters at the joint news conference Friday that the US was "continuing to draw down our embassy" in the Ukrainian capital of Kyiv and "will continue that process."

In Washington, the White House convened a meeting with several Cabinet members and senior national security officials in the Situation Room on Thursday night to discuss Russia's ongoing military buildup near Ukraine, two officials familiar with the matter told CNN. There have been several Situation Room meetings in recent weeks and months to discuss the brewing crisis, one of the sources said, but the meetings have grown more urgent in recent days as Russia has continued to move forces, weapons and logistical equipment into the area and increased troop readiness.

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