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

"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 President Vladimir Putin was responsible.

The step is the latest by NATO aimed at beefing up its defences after allies spearheaded by the United States rushed thousands of troops to eastern members as the Kremlin moved on Ukraine.

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

"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."