Aimed at practicing “repelling external aggression”, Moscow and Minsk have not disclosed how many troops are participating in the 10-day war games. However, the US and NATO estimate Russia has sent around 30,000 combat troops with weaponry and other military equipment to the former Soviet neighbor .

Tensions are reaching breaking point as Russia has amassed more than 100,000 troops around the Ukrainian border, raising strong suspicions that they intend a major military incursion.

The Russian Defence Ministry Press Service has today released footage showing tank and anti-aircraft systems rolling across heavy snow.

Some of the drills involve live fire, including anti-tank missiles and armoured infantry vehicles apparently shooting at targets.

Boris Johnson said at a news conference today: “This is probably the most dangerous moment, I would say, in the course of the next few days, in what is the biggest security crisis that Europe has faced for decades, and we’ve got to get it right.”

He was joined by Nato Secretary General Jens Soltenberg, who added: “This is a dangerous moment for European security. The number of Russian forces is going up. The warning time for a possible attack is going down.

“We are closely monitoring Russia’s deployment in Belarus, which is the biggest since the end of the Cold War.”

It is understood the exercise will end on February 20.

However, a recent US intelligence estimate, made last week, said Russia would have sufficient force for a full-scale invasion by February 15.

Speaking to France Inter radio, French foreign minister Jean-Yves Le Drian described the exercises as a “very violent gesture that concerns us”.

Russian and Belarusian military leaders said in a joint statement: “The purpose of the exercise is to work on methods to stop and repel external aggression within the framework of a defensive operation.”

The exercise has been dubbed Union-Courage 2022.

Volodymyr Zelensky, president of Ukraine, described the drills as “psychological pressure.”

He added, however, that Ukraine is used to the experience, saying: “The risks are there and have been there since 2014. The issue is the degree of these risks, and how we respond to them.”

The drills come during a period of intense diplomacy which has seen Foreign Secretary Liz Truss travelling to Moscow to meet her Russian counterpart, Sergei Lavrov.

Speaking after his talks with Ms Truss, Mr Lavrov said he could not understand British anxiety over the drills.

He said: “Some time soon, Western nations will find out the Russian-Belarusian drills wrapped up and that Russian troops came back to the Russian territory.

“They will then make a noise about the West forcing Russia over to de-escalate while it will be like selling air: Everyone knows well that Russian troops always come back to their bases at the end of all drills.”

He added that it was the sovereign right of any government to determine the length of military drills it chooses to hold.

Russia has also sent six warship through the Bosphorus for naval drills on the Black Sea and the neighbouring Sea of Azov.

Kyiv denounced their presence as “unprecedented” attempt to cut off Ukraine for both seas.

Weapons seen being set up include a D-30 Howitzer – a Soviet field gun – as well as combat crews on a S-400 air defence system.

The Russian-made S-400 is one of its most advanced anti-aircraft weapons, and can be seen in the footage driving along an icy, snow-covered road before forming a line and readying their payload.