• President Joe Biden has issued a warning that U.S. citizens should leave Ukraine immediately as tensions with Russia over its military activity continue to intensify.
• Secretary of State Antony Blinken on Friday also urged Americans to leave Ukraine, warning "an invasion could begin at any time."
• Meanwhile, British Prime Minister Boris Johnson warned on Thursday, "Things are as dangerous as I have seen them in Europe for a very, very long time."

President Joe Biden has issued a warning that U.S. citizens should leave Ukraine immediately as tensions with Russia over its military activity continue to intensify.

"American citizens should leave. … Leave now," Biden told NBC News' Lester Holt on Thursday night. "We're dealing with one of the largest armies in the world. This is a very different situation, and things could go crazy quickly."

Russia began a 10-day program of military exercises with its neighbor Belarus on Thursday. NATO estimated 30,000 Russian troops were taking part, marking Moscow's biggest military deployment in Belarus since the Cold War.

The drills, widely seen as a display of strength by Russia, come as around 130,000 soldiers, tanks, missiles and even fresh blood supplies have been moved to its border with Ukraine, according to NBC News. Russia is demanding that Ukraine never be permitted to become a member of the NATO military alliance and has said it wants the organization to roll back its presence in Eastern Europe.

Russia has insisted it has no plans to invade Ukraine.

Ukraine also accused Russia on Thursday of creating a sea blockade, the BBC reported. Ukrainian Foreign Minister Dmytro Kuleba reportedly said the Sea of Azov was completely congested, while the Black Sea was being blocked by Russian forces as they prepared for naval drills next week.

Despite the mounting tensions, Biden on Thursday ruled out sending U.S. troops into Ukraine, saying there was no scenario in which he could see this happening.  

"That's a world war when Americans and Russians start shooting at one another, we're in a very different world than we've ever been in," he said.

Biden added that it would be difficult to position troops in Ukraine even for the purpose of evacuating U.S. citizens.

"How do you do that? How do you even find them?" he said. "I'm hoping that 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."

Invasion 'could happen at any time'

On Friday, U.S. Secretary of State Antony Blinken told a news conference in Melbourne, Australia, that the government was continuing to "draw down" its embassy in Ukraine amid "very troubling signs of Russian escalation."

These signs included Moscow moving more troops to the Ukrainian border, Blinken said, reiterating the call for U.S. citizens to immediately leave Ukraine.

"We're in a window when an invasion could begin at any time, and to be clear, that includes during the Olympics."

The Winter Olympics are currently being held in China and are scheduled to run through to Feb. 20.

Although Russia has denied it is planning to invade Ukraine, the U.S., the U.K. and other countries have sent military hardware to Kyiv to help it defend itself in case of military aggression.

Europe 'on the edge of a precipice'

Western countries, including the U.S., U.K. and France, have been in talks with Russian ministers in recent weeks in a bid to resolve the tensions diplomatically.

British Prime Minister Boris Johnson suggested on Thursday, however, that the situation remained fragile.

"I think today, 10th of February 2022, we stand on the edge of a precipice and things are as dangerous as I have seen them in Europe for a very, very long time," he warned. "It is up to Vladimir Putin now to disengage and to de-escalate, and the way forward is diplomacy."

U.S. lawmakers have said they are devising the "mother of all sanctions" against Russia as a method of defending Ukraine that would be "crippling to [the Russian] economy." British and German ministers have also warned there will be economic consequences for Moscow if it takes any aggressive action against Ukraine.

During a press briefing last week, analysts at think tank Chatham House told reporters that 24% of Ukrainians surveyed in recent weeks said they were ready to physically defend Ukraine, on top of those who had already joined its growing volunteer army.  

Ukraine President Volodymyr Zelenskyy has warned that any military confrontation with Russia would result in a "full scale" war on European soil.

Last Friday, Ukraine's Kuleba told CNBC that the government had been briefed by the U.S. that Russia may use a fake video as grounds for an invasion.

"If you ask me if there is anything Russia couldn't do in order to provoke the war, my answer would be no," he said. "Everything is possible, and we should take every risk into account."

Putin has accused the U.S. of using Ukraine as a tool to "contain Russia."

— CNBC's Holly Ellyatt contributed to this article.