Armoured tanks and vehicles marked with the letter “Z” are headed to the Ukrainian border.
They have been marked with the distinctive white letter inside a square box, in an attempt to avoid friendly fire if war breaks out.
The entourage of military transport travelled towards Ukraine's border giving another signal of imminent war.
Armed forces experts said it mimicked UK and US tactics in the First Gulf War when forces sent to free Kuwait marked military vehicles with an upturned chevron.
A military source in the Ukrainian capital Kyiv told The Sun : “It would suggest final preparations are complete.
“It’s vital that any attacking force can be distinguished, particularly from the air where Russian forces will have complete control.
“The Ukrainians have very similar tanks and vehicles and will want to reduce the risk of friendly fire.”
Video footage from near the Ukrainian border displayed markings on tanks, fuel trucks and supply vehicles.
The military vehicle procession marked “Z” was estimated to be six and 25 miles from Ukraine as more than 200,000 troops ringed the country.
Diplomatic sources in Kyiv told The Sun it was “widely understood” Vladamir Putin had decided to launch an invasion.
Reports on social media yesterday showed video footage of the “Z” markings on vehicles.
The vehicles were on the move as Russia broke a promise to withdraw its forces from northern neighbour Belarus after military exercises.
Chief of Belarusian General Staff Alexander Volfovich denied such a pledge had been made and insisted the “exercise” had been extended until Monday.
There are signs that plans to invade Ukraine had "already begun" as explosions were reported in Donetsk on Saturday night.
Russia and Ukraine have been fragile since Kyiv began to grow closer to the West prompting Russia to annexe Crimea in 2014.
Firing has been reported across the region as Ukraine - while Russia and the separatists blame each other for the clashes.
Artillery shells are said to have fallen within Russia's borders, resulting in the death two of Ukrainian soldiers while explosions were reportedly heard throughout the Donbas.
Mass evacuations of separatist groups happened on Friday as Ukraine was accused of planning an attack and alleging Kyiv plotted to blow up a chemical plant.
Both countries have now, The Sun claims, shown the "full mobilisation" of their military forces.
Rising chaos in the Donbas, it is feared will be used as a pretext to move in Putin's troops.
In a new "stunt" the pro-Moscow rebel authority in Donetsk claimed to have held a Ukrainian spy who "confessed" to plans to overrun the Donbas.
Anton Matsanyuk “confirmed that Kyiv intends to use all its strike power in the forcible seizure of the Donbas," said a report.
He confirmed a Ukrainian plan to invade Donetsk and Luhansk.
Prime Minister Boris Johnson, meanwhile, warned Russia is planning the "biggest war in Europe since 1945" as Putin intends to "invade and encircle Kyiv."
Jens Stoltenberg, NATO chief said: "Every indication indicates that Russia is planning a full-fledged attack against Ukraine.
"We all agree that the risk of an attack is very high."
Johnson also told world leaders an invasion of Ukraine was looking "increasingly likely" and later said the attack appears to already be "in motion".
"If Ukraine is invaded, the shock will echo around the world," said the PM in a chilling warning during his speech at the Munich Security Conference.
Asked if the Russian leader had made up his mind to attack the capital city Kyiv, he told broadcasters: "I think certainly things are in motion.
"But the question is whether it can all be pulled back and whether the President of Russia is still able to call this operation off."
He added: "Imagine the invasion of Ukraine, a country of 45 million people, the second biggest country in Europe geographically apart from Russia itself - an absolutely colossal place.
"You can't hold it down. There will be a protracted, violent, bloody insurgency with the loss of life for young Russians, as well as for Ukrainians."
The PM also warned Putin that he faces decades of "bloodshed and misery" if his troops invade Ukraine.
