“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,” Boris Johnson, the prime minister, said on Thursday at a joint news conference with Nato Secretary General Jens Stoltenberg.

Mr Stoltenberg echoed those sentiments, saying: “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,” he added.

The purpose of the exercise is to work on methods to stop and repel external aggression within the framework of a defensive operation,” Russian and Belarusian military leaders said in a joint statement. The exercise has been dubbed Union-Courage 2022.

This picture shows a D-30 Howitzer being set up.