Nordic officials say that NATO planes are joining their air forces in one of Europe's largest fighter jet exercises, as military tensions increase in the Nordic and Baltic region.

The two-week Arctic Challenge Exercise started Monday with more than 100 aircraft and 4,000 people participating. It follows NATO submarine detection drills off the Norwegian coast earlier this month.

The aerial manoeuvers, hosted by NATO-member Norway, include U.S. F-16 fighter jets and military aircraft from Britain, Germany and France, accompanying non-NATO members Finland and Sweden.

Tensions have increased in the region since the Ukraine crisis. NATO is accusing Russian military aircraft of putting civilian flights at risk by boosting the number of its military air patrols and having its planes fly with their transponders switched off.