US President Joe Biden has said that he believes Russian leader Vladimir Putin has decided to invade Ukraine, but added that there is still room for diplomacy to defuse the crisis.
"As of this moment I'm convinced he's made the decision. We have reason to believe that," said Mr Biden - saying an attack could come in the next "weeks" or "days".
"Until he does, diplomacy is always a possibility," Mr Biden added.
Mr Biden also questioned whether it was a "wise choice" for his Ukrainian counterpart Volodymyr Zelensky to leave the country, after warning that a Russian invasion appeared imminent.
Mr Zelensky is due to travel to this weekend's Munich Security Conference.
"In the pursuit of a diplomatic solution, it may not be, it may be the wise choice. But it's his decision," Mr Biden said during a press conference.
Today, Russian-backed separatists packed civilians onto buses out of breakaway regions in east Ukraine, a shock turn in a conflict the West believes Moscow plans to use as justification for all-out invasion of its neighbour.
Warning sirens blared in Donetsk after it and the other self-proclaimed "People's Republic", Luhansk, announced the evacuation of hundreds of thousands of people to Russia, with women, children and the elderly going first.
Hours later, a jeep exploded outside the rebel government building in the city of Donetsk, capital of the region of the same name. Reuters journalists saw the vehicle surrounded by shrapnel, a wheel tossed away by the blast. Russian media said it belonged to a militia leader.
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The Ukrainian government said it was not planning any offensive or targeting civilians in any way.
Most of the several million civilians in the two rebel-held areas are Russian speakers, with many already granted citizenship by Moscow.
Within hours of the surprise announcement, families assembled to board buses at an evacuation point in Donetsk, where authorities said 700,000 people would leave.
The evacuation started after the simmering eastern Ukraine conflict zone saw what some sources described as the most intense artillery bombardment for years today.
Ukraine's government and the separatists traded blame.
Western countries have said they think the shelling, which began on Thursday and intensified in its second day, is part of an attempt by Russian President Vladimir Putin's government to create a pretext to justify an invasion of Ukraine.
Russia has said it has no intention to attack Ukraine and accused the West of irresponsible fear-mongering.
In Donetsk, many people continued with their daily lives this evening, with no signs of panic.
Ukraine was the most painful loss for Russia of the 14 former republics under its control prior to the 1991 break-up of the Soviet Union. Mr Putin, who called the break-up the greatest geopolitical catastrophe of the last century, has devoted his rule to restoring Russia as a global power and defying the West.
With war fears shaking markets and Europe engulfed in a diplomatic crisis, Russia said this week it had started withdrawing troops from the border near Ukraine.
But the United States said it had instead done the opposite: ramping up the force menacing its neighbour to between 169,000-190,000 troops, from 100,000 at the end of January.
"We see additional forces going to the border including leading edge forces that would be part of any aggression," US Secretary of State Antony Blinken told the Munich Security Conference.
Western countries fear a conflict on a scale unseen in Europe at least since the Yugoslav and Chechen wars of the 1990s, which killed hundreds of thousands of people and forced millions to flee.
Ukraine is the second biggest country in Europe by area after Russia, and home to 40 million people.
"This is the most significant military mobilisation in Europe since the Second World War," US ambassador Michael Carpenter told a meeting at the Vienna-based Organisation for Security and Cooperation in Europe.
Ukraine said Russia was planning staged attacks, including a faked video of a raid on a chemical plant, and falsely accusing it of provocations in the separatist areas.
"Ukrainian forces are not planning any offensive operations, and will not use weapons if this might threaten peaceful civilians," the Ukrainian military said.
A diplomatic source with years of direct experience of the conflict described today's shelling in east Ukraine as the most intense since major combat there ended with a 2015 ceasefire." They are shooting - everyone and everything," said the source.
The evacuation piled further pressure on Russia's rouble currency and other assets.
The West has threatened tighter economic sanctions on Russia if it invades Ukraine. Mr Putin, whose nation has already been under sanctions since 2014, told a news conference in Moscow that Western countries would probably find a reason to impose more sanctions whatever Russia does anyway.
Russia is making a set of security demands, including a promise to bar Ukraine from ever joining the US-European military alliance NATO, which the West says is any country's sovereign right. On Thursday, Russia sent a strongly-worded letter to Washington threatening unspecified "military-technical measures".
Since toppling a pro-Russian president in 2014, Ukraine has become closer politically to the West, staged joint military exercises with NATO and taken delivery of weapons including US Javelin anti-tank missiles and Turkish drones.
Kyiv and Washington see these as legitimate moves to bolster Ukraine's defence after Russia seized the Crimea region in 2014 and provided backing to separatists.
Mr Putin says Ukraine's growing ties with the alliance could make it a launchpad for NATO missiles targeted at Russia.
As Russia shows off its military capacity on several fronts, Mr Putin was to personally supervise exercises of its strategic nuclear missile forces on Saturday.
The Kremlin also has tens of thousands of troops staging exercises in Belarus, north of Ukraine. The drills are due to end on Sunday and Moscow has said the troops would go back to Russia at some point after.
The Russian-backed leader of Belarus, Alexander Lukashenko, met Mr Putin today, suggesting beforehand that the soldiers may remain.
