"This is dangerous rhetoric. This is a behavior which is irresponsible," the alliance's secretary-general Jens Stoltenberg told CNN amid heightened tensions over the Russian invasion of Ukraine.

"And, of course, if you combine this rhetoric with what they're doing on the ground in Ukraine, waging war against the independent sovereign nation, conducting a full-fledged invasion of Ukraine, this adds to the seriousness of the situation," he added.