Satellite images appear to show Russian forces massing near the border of Ukraine, raising fears of a war between the two neighbouring countries.
Footage shared on social media purports to show a Buk anti-aircraft missile system and army tanks in the Voronezh region, while naval war games are being undertaken in the tense Black Sea.
Space firm Maxar Technologies has also released images of supposed columns of Russian forces stationed in the Smolensk district.
Both Voronezh and Smolensk are less than 200 miles away from the Ukrainian border.
Intelligence reports have revealed that at least 90,000 Russian soldiers, along with heavy artillery and tanks, are now in place on the Russian side.
This could rise to as many as 175,000 personnel by early next year, reports claim.
The build-up comes as Vladimir Putin hit out at a journalist for a “provocative question” when asked if he intended to invade Ukraine, but in his answer the Kremlin president did not rule out doing so.
"Russia is pursuing a peaceful foreign policy, but it has the right to ensure its security," he said, following his video summit with Joe Biden.
He would be guilty of “criminal inaction” if he was to “watch impotently” a trend which has seen NATO deploy strike weaponry in Poland and Romania, with Moscow now concerned that the same could happen in Ukraine.
Russia has repeatedly claimed that moving troops and military hardware within its borders does not constitute intent to invade, as the West has alleged.
Admiral Sir Tony Radakin, the new chief of the defence staff, said a war between Russia and Ukraine could be the biggest battle in Europe for more than 75 years.
UK defence sources have told the Daily Mail that Russia’s plan would probably be to occupy eastern Ukraine in 2022 and form a pro-Russia state.
The military numbers of the Buk complexes reportedly seen in Voronezh region had been “crudely painted over”, according to the Conflict Intelligence Team (CIT), a Russian group which monitors military movements.
Such a tactic was also used on Russian military equipment which appeared in Crimea and eastern Ukraine in 2014 and 2015.
Western sources say it was a Buk complex in pro-Moscow rebel-held Donetsk region in Ukraine that downed Malaysia Airlines flight MH17, a passenger flight from Amsterdam to Kuala Lumpur seven years ago, killing all 298 on board.
New reports also allege that Msta-S self-propelled howitzers and T-80U tanks were seen at Maslovka railway station near Voronezh.
“This information allows us to conclude that despite negotiations between Biden and Putin, the concentration of Russian troops in the areas bordering territory controlled by the Ukrainian authorities continues,' the statement said.
The team claim analysis shows the transfer of new equipment in Voronezh region, as well as the movement of trains from Stavropol towards Crimea.
“In addition to troops, equipment is apparently being brought to the Ukrainian border, and in alarming quantities, which can only be explained at a stretch by the rearmament of active units,” the publication said.
It suggested that four trains spotted at Maslovka had come from the towns of Krasny Bor, Naro-Fominsk and Ilyino, bases of the 49th Anti-Air Missile Brigade, 4th Guards Tank Division and 288th Artillery Brigade.
These units are all part of the 1st Guards Tank Army, which allegedly has been building its strength in the area since November.
“This suggests the 1st Tank Army continues concentrating south of Voronezh, where it could threaten government-controlled regions of Ukraine,” stated the CIT.
“Some Western analysts …believe that even if an invasion is not ordered, that posture could be maintained through 2022.”
Meanwhile, the Black Sea Fleet has been involved in firing drills including patrol ship Sergei Kotov, based in port Novorossiysk.
"During a planned sea mission, the ship's crew successfully fired an AK-176 artillery gun, large-calibre shipborne machine guns and DP-65 anti-sabotage grenade launchers,” said a statement from the fleet reported by Interfax.
Separately, Black Sea Fleet engineering units countered "actions of sabotage performed by reconnaissance groups of a simulated enemy”, it was reported.
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"In total, more than 200 service personnel took part in the combat training event, about 50 pieces of military and special equipment were involved," said the fleet.
Putin voiced his concern at Ukraine joining NATO, despite no invitation having been issued to the ex-Soviet state.
"We are concerned over the prospects of Ukraine’s possible accession to NATO, as it will definitely result in the deployment of military contingents, bases and weapons posing a threat to us,” he said.
A video shows him saying: “We now witness (Western) missile defence systems in Poland and Romania.
“We have every reason to believe that the same will happen if Ukraine joins NATO but this time on Ukrainian soil
“How can we not think about that?
“It would be criminal inaction on our part to watch impotently what happens there.”
