Sources: Russia letter to NATO contains nuclear threat | NHK WORLD-JAPAN News

Multiple NATO sources say Russia has sent an informal letter to NATO, in which Moscow warned it is ready to use nuclear weapons if the alliance attempts to isolate the Russian exclave of Kaliningrad from the country's mainland.

Kaliningrad faces the Baltic Sea and borders two NATO nations -- Poland and Lithuania.

One of the NATO sources told NHK that the letter is seen as a Russian response to developments including a recent military exercise held near Kaliningrad by NATO countries.

The source noted it is unusual for Russia to send such a document to NATO and that the move may have been part of Moscow's hybrid attacks. This suggests that Russia may be testing NATO's solidarity through non-military means.

NATO Secretary General Mark Rutte said on Wednesday that the alliance has "reacted" to what Russia had sent and that it has urged Moscow to "stop the nuclear threats."

Rutte added that there is "no way" Russia can ever beat NATO because the alliance is "so much stronger" than the country.