"We cannot afford a security vacuum in the High North. It could fuel Russian ambitions, expose NATO and risk miscalculation and misunderstandings," Stoltenberg said.

"We also see an increased Chinese interest in the region. China has defined itself as a near Arctic state and aims to build a presence here," he added.

Stoltenberg was speaking during a visit to the Bardufoss base in northern Norway, where the alliance is conducting large-scale military exercises, dubbed Cold Response.

Moscow had increased military activity in the Arctic in recent years, he told a press conference, modernising its existing bases and building new ones in a clear sign it intends to be a dominant player in the coveted region.

Russia's Kola Peninsula, which borders Arctic Norway, is home to the powerful Northern Fleet, with its huge concentration of nuclear weapons and numerous military installations.

"For all these reasons the High North is an area of critical importance for all Allies. This is why NATO has increased its military presence in the North," said Stoltenberg, whose term of office has been extended for a year because of Russia's invasion of Ukraine, on NATO's eastern flank.

This month's Cold Response exercise in Norway is designed to test the ability of NATO members -- and partners Sweden and Finland -- to come to the aid of another member state in difficult climate conditions.

Some 30,000 soldiers are taking part in the air, sea and land exercises, the biggest manoeuvres Norway has organised since the end of the Cold War.

Planned long in advance, the exercise has taken on added significance following Russia's invasion of Ukraine.

"Cold Response is an important exercise, not least in light of the meaningless and senseless Russian attack on Ukraine," Stoltenberg said.

"We regret, of course, that Russia declined to observe that exercise, but we regret even more that Russia never invites us to take part in mandatory observation and inspection of their exercises," he continued.

"You have to remember that the war President (Vladimir) Putin now wages against Ukraine was in the beginning disguised as an exercise... and then suddenly exercise turned into a full-fledged war."

In addition to having huge strategic importance, the environmentally fragile Arctic contains substantial reserves of oil, gas and minerals.

And while climate change is reducing its sea ice cover with devastating effect, the ice melt opens up a shipping route between east Asia and Europe that is considerably shorter than the passage via the Suez Canal.