Finland and Sweden, both militarily non-aligned, are debating whether to seek NATO membership as a deterrent against potential Russian aggression in the wake of Moscow's invasion of Ukraine.

But the countries' leaders have acknowledged the Nordic states could be left open to Russian cyberattacks or territorial violations in the months before a bid is ratified.

NATO chief Jens Stoltenberg, speaking to reporters in Brussels on Thursday, said he was "certain that we will be able to find arrangements for that interim period between (the time when) Finland and Sweden apply and until the formal ratification is finalised."

Stoltenberg later spoke to Finnish President Sauli Niinisto in a telephone call.

"Stoltenberg again gave strong support for Finland's decision," Niinisto said in a statement, providing no other details about their discussion.

Niinisto said they would meet face-to-face soon.

On April 20, Finland's parliament began debating whether to seek NATO membership, after Russia's invasion of Ukraine sparked a surge in political and public support for joining the military alliance.

Many analysts believe the country's leaders will decide in mid-May on whether to join, which would allow for an application to be submitted before NATO leaders meet for a summit in June.