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.

The Finnish Prime Minister Sanna Marin said on Thursday in Athens that a decision on the country's NATO membership "will be taken very soon".

"Our message to NATO allies is this: If Finland will join NATO we will contribute to the security of the whole alliance," Marin said during a joint news conference with the Greek Prime Minister Kyriakos Mitsotakis.

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.

The Finnish PM said the parliament and the government, together with the president, will assess all the implications of the new security situation and will have to consider the question of whether to apply for membership.

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.

Any membership bid must be accepted by all 30 NATO states, a process that could take four months to a year.