According to NATO statement, the North Atlantic alliance is concerned about the Russian missile system that carries nuclear warheads which breaches the treaty of 1987 on medium-range nuclear forces. 

“The Allies have discovered that the Russian missile system is causing serious concern; NATO urges Russia to respond to this concern by acting thoroughly and transparently, as well as participating in a technical dialogue with the US,” the statement said.

The agreement on nuclear forces of medium range forbids missiles with a range of 500 to 5,500 kilometers.

"The US will act in accordance with its obligations under the treaty," NATO informed.

Earlier, American media outlets reported that Vladimir Putin, the Head of the Russian Federation, was quite likely to be able to store nuclear weapons in the territory of the occupied Crimea. Thereby, the occupied Crimean peninsula, could become a nuclear threat, which refers to one of the elements of the Russian so-called "hybrid war". According to Phillip Karber, the President of the Potomac Foundation, the US authorities posess the information indicating that an active transfer of nuclear weapons from the Russian territory to the Crimea is taking place.