NATO leaders have invited Russian observers to view Trident Juncture, the alliance's largest military exercise in over a decade.

The exercise, part of NATO's initiative to ramp up defenses in the face of a changing security environment, has been widely seen as a response to Russian military aggression.

Moscow authorities are currently in their most complex standing with NATO and the rest of the West following their push in Crimea, political and material support for militants in eastern Ukraine, and backing Syrian President Bashar al-Assad in his regime's fight against the Free Syrian Army. Officials from the United States and other NATO partner countries have publicly spoken out against Russian President Vladimir Putin's actions.

The training covers several platforms of combat. Over 70 naval ships and submarines are to be used for maritime activities. Rear Admiral Jorg Klein, the Commander of Standing NATO Maritime Group 2, praised the diversity of the participants in the 3-week excursion.

"I am glad to be in this challenging NATO exercise with my units and look forward to train with these highly motivated crews from different nations. Multi-nationality is inspiring; common procedures and tactics are the key to shaping a coherent and capable NATO force at sea," Klein said in a statement.

Trident Juncture 2015 involves around 36,000 personnel from more than 30 nations. The training concludes on November 6.