More than 9,000 troops, several warships and dozens of aircraft from Nato countries have been deployed in the Atlantic and Europe as part of a major military exercise.
NATO said Steadfast Defender 2021 was designed to prepare its forces for any threat and ensure they work co-operatively.
The Alliance insisted the three-part exercise was not aimed at Russia specifically, but tension with Moscow remains high.
Nato has stationed battlegroups in member states Estonia, Latvia, Lithuania and Poland – each of which borders Russia – in recent years to establish its “forward presence” in the Black Sea region.
While Nato is at odds with Russia on a number of issues, Moscow’s deployment in April of thousands of soldiers to its border with Ukraine caused alarm.
“Nato is there to defend all our allies, and this exercise sends a message about our ability to transport a large number of troops, equipment across the Atlantic, across Europe and also to project maritime power,” said Secretary General Jens Stoltenberg last week aboard the British aircraft carrier HMS Queen Elizabeth.
Russia, meanwhile, announced the formation of 20 new units in the west of the country. Defence Minister Sergei Shoigu said the move was in response to Nato’s frequent military drills, its deployment of warships and flights by US bombers near Russia territory.
He said such actions “destroy the international security system and force us to take the relevant countermeasures”.
On Tuesday, Mr Stoltenberg said Russia’s measures had led Nato to increase “the readiness” of its forces.
“Russia over the last years has invested heavily in new, modern military capabilities, from conventional to nuclear weapon systems” and “has been willing to use military force against neighbours in Georgia, in Ukraine”, he said.