Finnish President Sauli Niinisto and Prime Minister Sanna Marin expressed support for NATO membership -- in defiance of warnings from Moscow -- and a formal statement on joining is expected on Sunday.

"This is a sovereign decision by Finland, which NATO fully respects," Stoltenberg said.

"Should Finland decide to apply, they would be warmly welcomed into NATO, and the accession process would be smooth and swift."

Finland and Sweden have taken steps towards joining the alliance since western powers began reviewing their security structures in the wake of Russia's invasion of Ukraine.

The Kremlin denounced the idea that Finland, its neighbour, would join NATO.

"The expansion of NATO and the approach of the alliance to our borders does not make the world and our continent more stable and secure," Kremlin spokesman Dmitry Peskov told reporters.

Helsinki insists its decision is not a threat to anyone.

But Finnish Foreign Minister Pekka Haavisto told the European Parliament that "Russia's invasion of Ukraine has altered the European and Finnish security environment" and his country joining "would further strengthen NATO as a future ally".

Stoltenberg stressed it was Finland's decision to make, and not a NATO power grab.

"I agree with President Niinisto and Prime Minister Marin that NATO membership would strengthen both NATO and Finland's security," he said.

"Finnish membership would demonstrate that NATO's door is open, and that Finland decides its own future."

Separately, European Council chief Charles Michel welcomed EU member Finland's leaders "paving the way" for their country to join NATO.

He said the EU and NATO "have never been closer" and that Finland joining the alliance would be a "historic step, once taken, that will greatly contribute to European security.

"With Russia waging war in Ukraine, it's a powerful signal of deterrence," he tweeted.