The Ukraine crisis was at its most dangerous moment, and it was important to get the next few days right as intelligence on Russia's troop movements on Ukraine's border remained grim, British Prime Minister Boris Johnson said at a news conference with NATO Secretary-General Jens Stoltenberg in Brussels.

Russia's deployment for a military exercise in Belarus and on the borders of Ukraine marks a "dangerous moment", Stoltenberg said.

"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," Stoltenberg added.