SHOCK videos appear to show Russian tanks, armoured vehicles and troops massing near Ukraine amid fears Vladimir Putin may be preparing an invasion.
US officials have privately alerted their European allies about the possibility of an attack - which would be the culmination of a long-running bitter feud with Ukraine.
Footage shared on social media platforms appears to show staggering amounts of Russia military gear parked up near the city of Voronezh.
Voronezh is around 200 miles east of the Ukrainian border and intelligence firm Janes reported a large number of Russian tanks, howitzers and armoured vehicles had been moved there.
The vehicles are reported to have been shipped down from the Moscow area and includes a battalion of T-80U main battle tanks.
Other videos show a group of tanks being carried by a train, also near to the city, on Thursday.
And further footage shows a line of military troop-carrying trucks steaming down a highway in Bryansk around 100 miles from the border at the end of October.
US secretary of state Antony Blinken has described Washington as "very concerning" - but Russia has rubbished the reports and insisted it means no harm.
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Whitehall sources said the UK government was also worried about the intel and there was “twitchiness” and “anxiety” among officials.
And meanwhile, French officials also said they were extremely concerned about the latest flare up in Eastern Europe.
The US intelligence is believed to be based upon details that have not yet been disclosed with Europe.
US officials have now urged Moscow to reconsider making a "serious mistake", spurred by concerns from US President Joe Biden.
Russia has always insisted it means no harm to Ukraine.
The patterns of Russian behaviour are different from what we have seen before
NATO source
"Such headlines do nothing more than pointlessly and groundlessly fuel tensions. Russia does not pose a threat to anyone," Putin's spokesman Dmitry Peskov said.
Putin has been repeatedly accused of pointing his military at Ukraine ever since Russia illegally annexed Crimea from the former Soviet state in 2014.
"The patterns of Russian behaviour are different from what we have seen before," a NATO source said.
"So far, it is unclear if this military build-up is intended to lead to an incursion into Ukraine or if it is just another exercise."
Vladimir Putin has already come under fire after being accused of creating a migrant crisis in the Poland-Belarus border to destabilise Europe.
His country deployed two nuclear bombers to patrol the skies over Belarus amid the bitter border dispute.
Russia also claimed to have chased off a British spy plane on Thursday over the Black Sea region.
And meanwhile, British fighter jets intercepted a pair of Russian bombers as they flew over the North Sea.
British troops have also been sent to help on Poland's border with Belarus amid the growing migrant crisis which is feared to be fuelled by Putin.
Poland’s defence minister, Mariusz Blaszczak, said British Royal Engineers are undertaking a "reconnaissance" mission alongside Polish forces. .
“Our soldiers will co-operate in strengthening the fence of the Polish-Belarusian border," said Mr Blaszczak.
The Belarusian military said the exercise involving a battalion of Russian paratroopers was intended to test the readiness of the allies’ rapid response forces due to an “increase of military activities near the Belarusian border”.
What is happening between Russia and Ukraine?
RUSSIA and the Ukraine have remained technically at war since 2014.
Ukraine was aligned with Russia as part of the Soviet Union until its collapse in 1991, following which it became an independent state.
Both nations remained closely tied - but Ukraine gradually began to distance itself, seeking deeper ties with the West.
The open conflict was triggered by the Ukrainian Revolution in 2014 - when an uprising overthrew the pro-Russian government of Viktor Yanukovych.
Vladimir Putin's forces reacted by annexing the region of Crimea from Ukraine - a move which was widely condemned by the West.
The conflict then spiralled when pro-Russian groups in Eastern Ukraine then took up arms against the state.
Russia gave their backing the separatist forces which formed breakaway republics in Donetsk and Luhansk.
Putin's forces then launched a military incursion into these regions as they gave their support to the rebels.
Russia continues to hold Crimea - and claims the region joined them willingly after they a referendum.
Seven years have now passed and the War in Donbass remains at a stalemate.
It is estimated some 14,000 have been killed in the conflict, including more than 3,0o0 civilians.
Ukraine and the rebels signed a new ceasefire in July 2020 - but clashes have been steadily increasing again throughout 2021.
Russian officials have insisted said the troops had only been deployed to the border for maneuvers and in response to the increased NATO activity near borders.
The Kremlin previously warned the West was “playing with fire” by deploying warships in the Black Sea and warned of the “risks of a clash”.
They complained that two US warships and four NATO spy planes had been sent close to Crimea in just 24 hours.
Earlier this week, US Secretary of State Antony Blinken reassured Ukraine that the US commitment to ensuring their security is "ironclad".
Directly referring to the build up of Russian forces, he warned any "escalatory or aggressive actions would be of grave concern to the United States."
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The West insists that under international law Crimea belongs to Ukraine.
Earlier this year, fighting over the territory triggered a major diplomatic incident as Putin sent tens of thousands of troops and weaponry to the border.