Moscow is seeking to create a "zone of influence through military means", NATO chief Jens Stoltenberg said Thursday, adding that the alliance has observed major and aggressive manoeuvres on the Russian side.

"We are observing massive militarisation at NATO borders -- in the Arctic, in the Baltic, from the Black Sea to the Mediterranean Sea," Stoltenberg told Germany daily Bild in an interview.

"Russia is trying to build up a zone of influence through military means," he said.

"We are registering aggressive, unannounced, large-scale manoeuvres on the Russian side. Therefore we must act," said Stoltenberg, justifying the alliance's decision to deploy battalions to the Baltic states and Poland.

"What we are doing is defensive, we do not want to provoke conflict, rather, we want to prevent conflict. We want to show our partners that we're there when they need us," added the NATO secretary-general.

Russia's 2014 annexation of Crimea and its support for separatists in Ukraine has jolted NATO out of a post-Cold War complacency and forced it to bolster its eastern flank.

NATO defence ministers on Tuesday approved sending four battalions of between 800 to 1,000 troops each to the three Baltic states and Poland just weeks before a landmark summit in Warsaw endorses a major build-up to counter a more assertive Russia.

Russia bitterly opposes NATO's expansion into its Soviet-era satellites and last month said it would create three new divisions in its southwest region to meet what is described as a dangerous military build-up along its borders.