(QHA) -

NATO's top commander says Russia is reinforcing its bases in Crimea, but there is no clear assessment of whether that includes deployment of nuclear weapons.

U.S. Gen. Philip Breedlove said Tuesday that Russian forces "capable of being nuclear" are being moved to the Crimean Peninsula, but NATO doesn't know if nuclear weapons are actually in place.

Last week, Breedlove said Russia was discussing plans to put aircraft in Crimea, which Moscow annexed from Ukraine earlier this year.

NATO has reported a spike in Russian military flights over the Black, Baltic and North seas and the Atlantic Ocean, with groups of Russian warplanes conducting large-scale maneuvers in international airspace.

A study by the European Leadership Network offered details on 40 incidents and said that they add up to a disturbing pattern over a wide geographic area.