"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.