The exercises are aimed at working out the reaction of the alliance in the event of an attack on any of its members
Thousands of NATO troops, several warships and dozens of aircrafts are taking part in military exercises that take place in Europe and the Black Sea region.
The exercise, dubbed Steadfast Defender, aims to train the alliance's response in the event of an attack on any of its members.
This will test NATO's ability to redeploy troops from America and maintain supply lines.
Russia's decision to send thousands of troops to the border with Ukraine last month raised concerns among the military alliance, which launched one of the largest defense spending initiatives since Russian troops annexed the Ukrainian peninsula of Crimea in 2014.
NATO senior officials insist that the military exercises, which involve some 9,000 troops from 20 countries, are not specifically aimed at Russia, but are focused on the Black Sea region, where Russia, according to the alliance, restricts freedom of navigation.
NATO Secretary General Jens Stoltenberg says the exercise is an important signal of the alliance's readiness for any potential adversary.