The United Kingdom and the United States advised their nationals to leave Ukraine on Friday amid the possibility of a military operation by Russia.

The Foreign Office "now advises against all travel to Ukraine. British nationals in Ukraine should leave now while commercial means are still available," it said in an update on its website.

A Russian invasion of Ukraine "could begin at any time" including during the Beijing Winter Olympics, U.S. National Security Adviser Jake Sullivan said Friday.

"We continue to see signs of Russian escalation including new forces arriving at the Ukrainian border," Sullivan told reporters at the White House.

"An invasion could begin at any time should Vladimir Putin decide to order it," he said. "It could begin during the Olympics despite a lot of speculation that it will only happen after the Olympics," he added.

Sullivan told nationals in Ukraine to leave in the next 24-48 hours or as soon as possible as there may be no future opportunities to leave if Russia invades

Meanwhile, British Prime Minister Boris Johnson on Friday told Western allies that "he feared for the security of Europe" due to the ongoing crisis on the Russia-Ukraine border, his Downing Street office said.

Johnson joined a call with United States President Joe Biden and other world leaders, telling them "that he feared for the security of Europe in the current circumstances."

Britain's defense secretary visited Moscow on Friday in another effort to ease tensions over a possible invasion of Ukraine by Russia, which held massive war games near its neighbor and insisted that the standoff with the West wasn't "our fault."

Ben Wallace's trip came a day after British Foreign Secretary Liz Truss held frosty negotiations in Moscow to urge Russia to pull back over 100,000 troops near Ukraine. Her Russian counterpart scathingly described the talks as a "conversation between deaf and dumb."

Russia says it has no plans to invade but wants the West to keep Ukraine and other former Soviet countries out of NATO. It also wants NATO to refrain from deploying weapons there and to roll back alliance forces from Eastern Europe – demands flatly rejected by the West.

Speaking at the start of his talks with Wallace, Russian Defense Minister Sergei Shoigu noted that "the military-political situation in Europe is growing increasingly tense, and it's not our fault."

He noted that shipments of weapons to Ukraine by the U.S., Britain and other allies have contributed to the tensions and pointed to the recent deployment of British soldiers to Ukraine, asking why they were sent and how long they will stay.

Speaking to reporters after the talks, Wallace noted that the anti-tank missiles that Britain sent to Ukraine were defensive tactical weapons that do not pose a threat to any neighbor unless it invades.

He said British troops deployed to Ukraine to help train its military to use the British weapons and will leave "pretty soon" after they accomplish that mission.

Wallace described the talks as "constructive and frank" and noted his Russian counterpart's assurances that Moscow has no intention to attack Ukraine. But he also emphasized that the concentration of Russian troops near Ukrainian territory is clearly "beyond normal exercising," explaining that about half of Russia's land forces are concentrated around the border with Ukraine.

He reaffirmed that a Russian invasion would have "tragic consequences" and emphasized the need to maintain contacts between military forces to prevent incidents.

"What is incredibly important, especially at this time with over 100,000 troops at high readiness on the borders of another country, is that we do not get into a position of miscalculation or escalation," Wallace said. He stressed that only through "the ability to talk to each other at times of concern can we pave the way for any de-escalation measures."

In an interview Thursday with NBC News, U.S. President Joe Biden repeated his warning that any Americans still in Ukraine should leave as soon as possible.

"It’s not like we’re dealing with a terrorist organization. We’re dealing with one of the largest armies in the world. It’s a very different situation, and things could go crazy quickly," he said.

Biden planned to hold a call with trans-Atlantic leaders later in the day.

Asked whether there were any scenarios that would prompt him to send U.S. troops to Ukraine to rescue Americans, the president said: "There’s not. That’s a world war when Americans and Russia start shooting at one another."

Speaking Friday on a visit to Australia, U.S. Secretary of State Antony Blinken did not detail the reasons behind the latest State Department’s security alert urging all American citizens to leave Ukraine.

"We’re in a window when an invasion could begin at any time and, to be clear, that includes during the Olympics," Blinken added. The Olympic Games are scheduled to end Feb. 20.

Ukrainian Foreign Minister Dmytro Kuleba played down the U.S. advice to Americans to leave, saying that Washington has made similar calls before. He noted that the situation remains volatile.

Continuing its military buildup near Ukraine, Russia has moved six amphibious assault vessels into the Black Sea, augmenting its capability to land marines on the coast.

Moscow has announced sweeping drills in the Black and Azov Seas in the coming days and closed large areas for commercial shipping, drawing a strong protest from Ukraine on Thursday.

Kuleba voiced hope that the West would react to the Russian move, saying that "we have engaged our partners to prepare a coordinated response."

Ukraine's military chief, Valerii Zaluzhnyi, reported to President Volodymyr Zelenskyy Friday that the authorities plan to quickly engage 1.5 million to 2 million people in training for the army reserve.

Russia’s troop concentration includes forces deployed on the territory of its ally Belarus for massive joint drills involving firing live ammunition. Those exercises entered a decisive phase Thursday and will run through Feb. 20. The Ukrainian capital is about 75 kilometers (47 miles) south of the Belarus border.

NATO has stepped up military deployments to bolster its eastern flank, with the U.S. sending troops to Poland and Romania.

The U.S. Navy said Thursday that it has deployed four destroyers from the U.S. to European waters. The Navy did not directly tie this deployment to the Ukraine crisis but said the ships provide "additional flexibility" to the U.S. 6th Fleet commander, whose area of responsibility includes the Mediterranean, and will operate in support of NATO allies.

NATO Secretary-General Jens Stoltenberg visited a military base in Romania, hailing the ongoing deployment of 1,000 additional U.S. troops that will nearly double their current number there. "This is a powerful demonstration of trans-Atlantic unity," Stoltenberg said.

Russian Foreign Ministry spokesperson Maria Zakharova responded by noting that "NATO keeps building up its presence near Russia's borders and exacerbates the situation around Ukraine to create a pretext for that."

Russia and Ukraine have been locked in a bitter conflict since 2014 when Ukraine’s Kremlin-friendly leader was driven from office by a popular uprising. Moscow responded by annexing Crimea and then backing a separatist insurgency in eastern Ukraine, where fighting has killed over 14,000 people.

A 2015 peace deal brokered by France and Germany helped halt large-scale battles, but regular skirmishes have continued, and efforts to reach a political settlement have stalled.