Satellite images captured in the past few days appear to show Russian military continuing to amass near the Ukrainian border from three directions.
The imagery, released by US-based technology company Maxar, indicates deployments to the east in Russia, the north in Belarus and the south in annexed Crimea.
Diplomatic efforts to avert war continue but fears are growing that Russian President Vladimir Putin could order an invasion in the coming days.
Moscow has deployed weaponry and an estimated 130,000 troops to the border, meaning an attack could happen "very, very quickly", according to UK armed forces minister James Heappey.
Speaking to CNN, Maxar senior director Stephen Wood said the company had identified more than 550 troop tents and hundreds of vehicles at the formerly disused Oktyabrskoye airfield, north of the Crimean capital Simferopol.
A new deployment of troops, vehicles and helicopters at the Zyabrovka airfield near the city of Gomel in Belarus has also been seen, he said. There appears to be a field hospital at the site.
And another "large deployment" has arrived at a training area near the Russian city of Kursk, Mr Wood added.
The UK Foreign Office is now telling British nationals in Ukraine, thought to number in the low thousands, to "leave now while commercial means are still available".
British ambassador Melinda Simmons is remaining in Kyiv with a "core team", but some embassy staff and their families were being withdrawn.
UK troops sent to train Ukrainians to use British-supplied anti-tank missiles will be leaving this weekend, Mr Heappey confirmed.
Speaking to BBC Breakfast on Saturday, he said: "We are now confident that the artillery systems, the missile systems and the combat air are all in place that would allow Russia to launch – at no notice – an attack on Ukraine.
"And on that basis I think it is our responsibility to share with UK citizens our view that they should leave the country immediately while commercial means are still available."
"Putin and his colleagues would very much like to be able to say what they may do is a consequence of Western aggression in Ukraine, so it's very important to us, to everybody frankly involved, that we're very clear we won't play an active part in Ukraine", he added.
US President Joe Biden and Mr Putin will discuss the crisis by phone on Saturday.
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