After a marathon debate lasting a day-and-a-half, 188 of 200 members of parliament voted in favour of NATO membership, a dramatic reversal of Finland's military non-alignment policy dating back more than 75 years.

"Tonight, we will sign the application letter for NATO," Foreign Minister Pekka Haavisto told public broadcaster YLE.

"Then possibly tomorrow, together with Sweden, the Finnish ambassador to NATO will submit the letter to NATO," he said.

Finland, which shares a 1,300-kilometre (800-mile) border with Russia, and Sweden have been rattled by Russia's invasion of Ukraine.

"Our security environment has fundamentally changed," Finnish Prime Minister Sanna Marin told parliament Monday at the start of the debate.

"The only country that threatens European security, and is now openly waging a war of aggression, is Russia," she said.

Finland spent more than a century as part of the Russian empire until it gained independence in 1917. It was then invaded by the Soviet Union in 1939.

Finns put up a fierce fight during the bloody Winter War, but were ultimately forced to cede a huge stretch of their eastern Karelia province in a peace treaty with Moscow.

According to public opinion polls, more than three quarters of Finns want to join the alliance, almost three times as many as before the war in Ukraine began on February 24.

Swedish public support has also risen dramatically, albeit lower than in Finland at around 50 percent.

Sweden announced its official intention to apply for NATO membership Monday and Foreign Minister Ann Linde signed the application letter Tuesday.

The turnaround is also dramatic in Sweden, which remained neutral throughout World War II and has stayed out of military alliances for more than 200 years.

Swedish Prime Minister Magdalena Andersson and Finnish President Sauli Niinisto are to meet US President Joe Biden in Washington Thursday to discuss their historic bids.

- Turkish objections -

While NATO Secretary General Jens Stoltenberg has repeatedly said the two countries would be welcomed "with open arms", Ankara has thrown a spanner in the works by announcing last-minute objections.

Any NATO membership bid must be unanimously approved by the alliance's 30 members.

Turkey's President Recep Tayyip Erdogan has threatened to block the alliance's expansion, accusing Helsinki and Stockholm of harbouring militants from the Kurdistan Workers' Party (PKK).

The PKK has waged a decades-long insurgency against the Turkish state.

Sweden has suspended any arms sales to Turkey since 2019 over Ankara's military operation in neighbouring Syria.

"We will not say 'yes' to those (countries) who apply sanctions to Turkey to join NATO," Erdogan said Monday.

"Neither of the countries has a clear stance against terror organisations," he said.

Niinisto said Tuesday he was optimistic Finland and Sweden would be able to secure Turkey's support for their NATO bids.

"I am confident that with the help of constructive talks, the situation will be resolved," Niinisto told Sweden's parliament in a speech.

Diplomatic sources told AFP Turkey blocked a NATO declaration Monday in favour of Sweden and Finland's membership.

Sweden and Finland have sent delegations to Turkey to meet with Turkish officials.