But the move has angered the Kremlin which is promising retaliatory steps of a "military-technical and other nature" to stop threats to Russia's security.

Finland, which shares a 1300km border and a difficult past with Russia, has gradually stepped up its cooperation with the North Atlantic Treaty Organisation as a partner since Russia annexed Crimea in 2014.

But until Russia's invasion of Ukraine, the Nordic country had refrained from joining in order to maintain friendly relations with its eastern neighbour.

"Finland must apply for NATO membership without delay," Niinisto and Marin said in a joint statement on Thursday.

"We hope that the national steps still needed to make this decision will be taken rapidly within the next few days."

The view among Finns on NATO has changed rapidly after Russia initiated what it calls a "special operation" in Ukraine.

Russia said on Thursday that Finland's announcement on joining NATO was a "radical change" in its foreign policy, and that Moscow would respond.

"Russia will be forced to take retaliatory steps, both of a military-technical and other nature, in order to stop threats to its national security arising," the Russian foreign ministry said in a statement.

"Helsinki must be aware of the responsibility and consequences of such a move," the statement said.

Russia opposes the expansion of NATO, which it says is designed to constrain Russia. Prior to invading Ukraine in February, Moscow was demanding binding guarantees from the United States and NATO that the alliance would not admit new members.

Finnish public support for joining NATO has risen to record numbers over recent months, with the latest poll by public broadcaster YLE showing 76 per cent of Finns in favour and only 12 per cent against, while support for membership used to linger at only around 25 per cent for years prior to the war in Ukraine.

While military non-alignment has long satisfied many Finns as a way of staying out of conflicts, Russia's invasion of sovereign Ukraine has led an increasing number of them to view friendly relations with Russia as an empty phrase.

Ukraine's fate has been particularly disturbing for Finland to watch as it fought two wars with Russia between 1939 and 1944, repelling an attempted invasion but losing around 10 per cent of its territory in the subsequent peace agreement.

Finland's rapid shift towards NATO is likely to pull along neighbouring Sweden.

Sweden's ruling Social Democrats are expected to decide on Sunday whether to overturn decades of opposition to NATO membership, a move that would almost certainly lead to Sweden also asking to join the 30-nation alliance.

Russia has repeatedly warned both countries against joining the alliance. As recently as March 12 its foreign ministry said "there will be serious military and political consequences" if they do.

The speed of the Finnish decision to apply has come as a surprise to many, with most political discussions taking place behind the scenes out of fear over Russia's reaction.

In March, Finland's government initiated a security policy review and delivered a report for parliament to discuss in April, while also holding discussions with all parliamentary groups to secure backing for the decision to join the treaty.

In parallel with the domestic process, Finland's president and prime minister have been touring NATO's 30 existing member countries to win their support for Finland's membership.

NATO Secretary General Jens Stoltenberg has previously said it would be possible to allow Finland and Sweden to join "quite quickly".

Australian Associated Press