Nato and its allies opened their largest military exercise in more than a decade today, choosing the central Mediterranean to showcase strength in the face of threats from Russia's growing military presence, spanning from the Baltics to Syria.

As planes flew over a gala ceremony for Nato's military and political leaders at an air base in Trapani, southern Italy, Russia's resurgence was the central topic of concern, pushing NATO to embark on its biggest modernization since the Cold War.

"The last time Nato regularly held exercises of this magnitude, we were in the midst of the cold war facing the Soviet threat," Nato Deputy Secretary General Alexander Vershbow told the ceremony.

"Now we have a far more unstable and potentially more dangerous situation," he continued to say. "In the east, Russia has illegally annexed Crimea, it continues to support the separatists in eastern Ukraine, and it now appears to have entered the war in Syria firmly on the side of (Syrian President) Assad."

Looming over the exercises involving some 36,000 troops, ships and aircraft over five weeks is the issue of how far Russia's growing military presence from the Baltics to Syria can limit NATO's ability to move around by sea and air.

NATO officials insist the alliance is ready and able to defend any of its 28 allies from any threat, while its combined strength is far greater than Russia's military capacity.

Still, privately Nato officials and diplomats worry the alliance needs to be more agile to combat multiple threats beyond its borders, a fact underscored by Libya's collapse, the rise of Islamic State militants and Syria's civil war.