British Defence Secretary Ben Wallace speaks to Russian Defence Minister Sergei Shoigu. Photo / AP
The White House said that a Russian invasion of Ukraine could come within the week, possibly within the next two days, even before the end of the Winter Olympics, and urged Americans to leave the country now.
US national security adviser Jake Sullivan said the United States did not have definitive information that an invasion has been ordered by Russian President Vladimir Putin. But he said all the pieces were in place for a major military operation that could start "rapidly".
"The risk is high enough and the threat is now immediate enough that prudence demands that it is the time to leave now," Sullivan said.
"We are not saying that a decision has been taken by President Putin," Sullivan said. "What we are saying is that we have a sufficient level of concern based on what we are seeing on the ground, and what our intelligence analysts have picked up, that we are sending this clear message."
Britain's defence secretary, meanwhile, is visiting Moscow in another effort to ease tensions over a possible invasion of Ukraine by Russia.
Russia is holding massive war games in neighbouring Belarus and insisting that the highly strained relations are not its fault. Russia says it has no plans to invade but wants the West to keep Ukraine and other former Soviet countries out of Nato.
Britain's defence 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 neighbour 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 more than 100,000 troops near Ukraine. Her Russian counterpart scathingly described the talks as a "conversation between deaf and dumb".
Russia says 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 Defence 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 US, 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 the anti-tank missiles Britain sent to Ukraine were defensive tactical weapons that do not pose a threat to any neighbour 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 emphasised 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 emphasised 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 on Thursday with NBC News, US 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 organisation. 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 very soon.
Asked whether there were any scenarios that would prompt him to send US 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 on Friday on a visit to Australia, US 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 Winter Olympic Games are scheduled to end on February 20.
Ukrainian Foreign Minister Dmytro Kuleba played down the US advice to Americans to leave, saying 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 the West would react to the Russian move, saying that "we have engaged our partners to prepare a co-ordinated response".
Ukraine's military chief, Valerii Zaluzhnyi, reported to President Volodymyr Zelenskyy on 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 on Thursday and will run through to February 20. The Ukrainian capital is about 75 kilometres south of the Belarus border.
Nato has stepped up military deployments to bolster its eastern flank, with the US sending troops to Poland and Romania.
The US Navy said on Thursday that it has deployed four destroyers from the United States to European waters. The Navy did not directly tie this deployment to the Ukraine crisis but said the ships provide "additional flexibility" to the US Sixth 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 1000 additional US troops that will nearly double their current number there. "This is a powerful demonstration of trans-Atlantic unity," Stoltenberg said.
Russian Foreign Ministry spokeswoman 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 more than 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.
More than 60 fighter jets hit alleged missile production sites in Tehran.