Some 450 U.S. and Romanian troops and technical staff are taking part in joint military exercise in northwestern Romania flying U.S. F-16 fighter jets alongside Romanian Mig-21 LanceRs.

Four F16s and one LanceR took off from the military base Thursday as Dacian Viper 2014 got underway. The weeklong exercise, the fourth of its kind, was planned before Russia's recent annexation of Crimea.

NATO's eastern nations have been calling on the allies to beef up their presence following the annexation and tensions following that move.

NATO has already reinforced its Baltic air patrols and is performing daily AWACs surveillance flights over Poland and Romania.