Finland and Sweden on Wednesday handed in their bids to join the US-led alliance NATO, after Russia's invasion of Ukraine up-ended decades of military non-alignment.

"The applications you have made today are an historic step. Allies will now consider the next steps on your path to NATO," NATO chief Jens Stoltenberg said, after receiving the bids from the Finnish and Swedish ambassadors at the alliance's headquarters.

The membership push could represent the most significant expansion of NATO in decades, doubling its border with Russia, and President Vladimir Putin has warned it may trigger a response from Moscow.

But the applications face resistance from NATO member Turkey, which has threatened to block them over accusations the Nordic neighbours act as safe havens for armed groups opposed to Ankara.

"The security interests of all allies have to be taken into account and we are determined to work through all issues and reach rapid conclusions," Stoltenberg said.

"All allies agree on the importance of NATO enlargement. We all agree that we must stand together and we all agree that this is an historic moment which we must seize."

Helsinki, Stockholm and the other Western allies remain optimistic they can overcome Turkey's objections.

NATO ambassadors are expected to discuss the applications on Wednesday and could give the green light on opening formal talks with the pair on their bids.

Several NATO allies, most notably Britain, have offered security assurances to Finland and Sweden during the application period before they are covered by alliance's mutual defence pact.

"Over the past few days we have seen numerous statements by allies committing to Finland and Sweden's security. NATO is already vigilant in the Baltic Sea region and NATO and allies forces will continue to adapt as necessary," Stoltenberg said.

Finnish president 'optimistic' of NATO deal with Turkey
Stockholm (AFP) May 17, 2022 –

Finnish President Sauli Niinisto said Tuesday he was 'optimistic' Finland and Sweden would be able to secure Turkey's support for their NATO membership bids, despite Ankara's last-minute objections.

"I am confident that with the help of constructive talks, the situation will be resolved", Niinisto told Sweden's parliament in a speech, later adding during a question period: "I'm optimistic".

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, which 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 security organisation NATO," Erdogan said Monday.

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

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

Sweden and Finland have been caught off-guard by Ankara's objections, with both saying they had previously received positive signals from all NATO countries.

Niinisto said that during a conversation with Erdogan a month ago, the Turkish leader said his country "was favourable" to Finland's bid.

And then "this past week he said 'not favourable'."

A diplomatic source said Turkey blocked a NATO declaration on Monday in favour of Sweden and Finland's membership and that Erdogan alone took the decision.

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

"They say they will come to Turkey on Monday. Will they come to persuade us? Excuse us, but they shouldn't bother," Erdogan responded.

The two Nordic countries have failed to respond positively to Turkey's 33 extradition requests over the past five years, Turkish justice ministry sources told the official Anadolu news agency on Monday.

The agency reported Turkey wanted individuals that were either accused of having links to Kurdish militants or belonging to a movement blamed for the 2016 attempted overthrow of Erdogan.

Turkey has rebuked Stockholm especially for showing what it describes as leniency towards the Kurdistan Workers' Party.