Russian President Vladimir Putin's move to place his nuclear forces on high alert is dangerous and adds to his aggressive pattern involving Ukraine, NATO Secretary-General Jens Stoltenberg said Sunday.

Putin's decision to place his nuclear forces on alert amounts to "irresponsible" behavior, Stoltenberg also charged.

"This is dangerous rhetoric. This is a behavior which is irresponsible," 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.

"This is dangerous rhetoric. This is a behavior that is irresponsible. And of course, when you combine this rhetoric with what they are doing on the ground in Ukraine – waging war against an independent, sovereign nation, conducting full-fledged invasion of Ukraine – this adds to the seriousness of the situation," Stoltenberg said on CNN's "State of the Union" program.

"NATO does not want war with Russia, we don't seek confrontation," he told BBC World.

"We are (a) defensive alliance, but we need to make sure that there's no room for misunderstanding, miscalculation about our ability to defend and protect allies."

NATO is to deploy its rapid response force for the first time to bolster its eastern flank in the face of Russia's invasion of Ukraine.

The United States and Europe "need to really stand together ... to both the aggressive actions of Russia against Ukraine but also the threatening rhetoric coming from Moscow," Stoltenberg said.

"That's exactly what we do by strengthening NATO's military presence in the east."

Stoltenberg said Russia was "conducting a full-scale military invasion of a sovereign, peaceful nation" and that there was "no doubt" that Putin was responsible.

The step is the latest by NATO aimed at beefing up its defenses after allies spearheaded by the U.S. rushed thousands of troops to eastern members as the Kremlin moved into Ukraine.

But NATO's "main responsibility and core task" remains to prevent an attack against a member country, Stoltenberg said.

"Ukraine is a highly valued partner, we support them, but Ukraine is not covered by the same type of security, absolute security ... that applies for NATO allies."