Ukrainians were warned Wednesday could be the day they are invaded - this is how they responded, as Global Security Editor Rohit Kachroo reports
There is "little evidence" of Russia pulling back from the Ukrainian border, Boris Johnson has said.
Reports suggested the Russian military could launch an invasion of Ukraine on Wednesday, despite President Vladimir Putin's claims on Tuesday that forces were starting to withdraw.
ITV News has examined footage that shows Russian tanks moving towards the border, rather than away, and there is significant doubt the Kremlin's words reflect reality.
In a call with United Nations secretary-general Antonio Guterres, Downing Street said: “The prime minister said there is currently little evidence of Russia disengaging, and they agreed any invasion of Ukraine would have catastrophic and far-reaching consequences.
“The leaders reiterated states’ responsibility to abide by their obligations under the United Nations charter and to respect the sovereignty and territorial integrity of UN members.
“They agreed to continue working closely together to pursue an urgent diplomatic resolution and avert a disastrous military escalation and humanitarian crisis.”
ITV News has studied video showing movement towards the Ukrainian border, not away
Britain’s head of military intelligence echoed the widespread scepticism over Russian claims of de-escalation, warning its forces are moving towards Ukraine.
Lieutenant General Sir Jim Hockenhull, the Chief of Defence Intelligence, said there had been sightings of additional armoured vehicles, helicopters and a field hospital.
“Contrary to their claims, Russia continues to build up military capabilities near Ukraine,” he said in a statement.
“Russia has the military mass in place to conduct an invasion of Ukraine.”
Nato secretary-general Jens Stoltenberg agreed there is still a "massive invasion force" ready to attack Ukraine.
Following a meeting with Nato representatives on Wednesday to discuss how invasion can be prevented, he said: "So far we do not see any signs of de-escalation on the ground - no withdrawals of troops or equipment.
"This may, of course, change however, what we see today is that Russia maintains a massive invasion force ready to attack, with high-end capabilities from Crimea to Belarus.
"This is the biggest concentration of forces in Europe since the Cold War."
Mikk Marran, director general of the Estonian Foreign Intelligence Service, speaking in connection to the publication of its annual report, told ITV News "Russia is ready to attack at any moment."
“We are now counting altogether between 150,000 and 170,000 troops close to the borders… we see approximately ten more BTGs (battalion tactical groups) moving towards the border.”
An official said Russian intelligence had prepared strategically important targets and described the build up of forces as “unprecedented.”
The official described three possible next steps for Russia: an intensification of fighting in Donbas, which offered Russia “plausible deniability”; a limited military attack; or a full-scale operation - but there are “not enough forces yet” to carry out the operation.
Earlier on Wednesday, however, Russia mocked the West over claims an incursion is imminent.
Russian Foreign Ministry spokeswoman Maria Zakharova told a TV press conference: "Sorry for being a little late, I was just checking whether we are invading Ukraine or not – spoiler: we are not."
The spokeswoman, who has taunted Western nations in the past, went further by asking the UK and US intelligence services for a Russian invasion schedule so she could plan her holidays.
In a Facebook post she asked the West to “to reveal the schedule of our ‘invasions’ for the upcoming year. I’d like to plan my vacations.”
Ms Zakharova claimed on Tuesday that Russia had "humiliated and destroyed" the West after claiming it had withdrawn some troops from the border with Ukraine.
Russia’s ambassador to the EU accused the West of "slander" over claims of an impending invasion, telling German newspaper Welt "there won’t be an attack this Wednesday". “Wars in Europe rarely start on a Wednesday," added Vladimir Chizhov.
President Putin said on Tuesday he "of course" does not want another war, and that he was open to further dialogue with the US and its NATO allies.
Where are Russian troops, and how many are there?
The president received the backing of one of his few international allies on Wednesday, as he held talks with Brazil president Jair Bolsonaro.
Mr Bolsonaro told Mr Putin that Brazil is "in solidarity" with Russia, "we have much to collaborate on, in several areas, defense, oil and gas, agriculture, the meetings are ongoing".
He added: "We are available and I'm certain this will be a very profitable meeting for our people."