The leader of NATO-member Turkey spoke ahead of expected confirmations from the Nordic nations on Sunday that they will apply to join the Western military alliance.

Erdogan accused both countries of harbouring "terrorist organisations" in his unfavourable assessment of the membership bids.

"We do not have a positive opinion," Erdogan told journalists after Friday prayers in Istanbul.

"Scandinavian countries are like a guesthouse for terror organisations," he said.

Turkey has long accused Nordic countries, in particular Sweden, which has a strong Turkish immigrant community, of harbouring extremist Kurdish groups as well as supporters of Fethullah Gulen, the US-based preacher wanted over the failed 2016 coup.

Erdogan cited a "mistake" made by Turkey's former rulers who greenlit Greece's NATO membership in 1952.

"We, as Turkey, do not want to make a second mistake on this issue," he said.

- Needs unanimous support -

Moscow's February 24 invasion of Ukraine has swung political and public opinion in Finland and Sweden in favour of membership as a deterrent against Russian aggression.

Both countries have long cooperated with NATO, and are expected to be able to join the alliance quickly.

NATO chief Jens Stoltenberg has repeatedly said they would be welcomed "with open arms".

Turkey's "not positive" response is the first dissenting voice at the prospect of the two Nordic countries' NATO prospects.

Stockholm and Helsinki have cranked up their international contacts to seek support for their potential bids.

Once a country has decided to apply for NATO membership, the 30 members of the alliance must agree unanimously to extend a formal invitation, which is followed by membership negotiations.

The final approval could then take place at a NATO summit in Madrid at the end of June. The 30 member states would then have to ratify the decision.

Turkey, which enjoys good relations with Kyiv and Moscow, has been keen to play a mediating role to end the conflict and has offered to host a leaders' summit.

Ankara has supplied Ukraine with combat drones, but has shied away from slapping sanctions on Russia alongside Western allies.

- 'Hungary of the EU' -

Turkey's position on Sweden and Finland's NATO membership risks making it look like the "Hungary of the EU", said Washington Institute fellow Soner Cagaptay.

Pro-Russia Hungary often breaks from its EU colleagues on a broad range of issues, including rule of law and human rights.

Cagaptay said Ankara should have negotiated its terror-related concerns behind closed doors with the two countries.

"The fact that this is done publicly is going to hurt Ankara's image significantly," he said.

Erdogan's comments may also raise tensions with France, whose President Emmanuel Macron has said that NATO was undergoing "brain death" partly due to Turkey's behaviour.

Macron has made clear he supports Finland's bid.

Finnish President Sauli Niinisto spoke with Erdogan in April as part of consultations for its NATO bid.

"I thanked President Erdogan for his efforts for peace in Ukraine. Turkey supports Finland's objectives," he tweeted at the time.

Turkish Foreign Minister Mevlut Cavusoglu will participate in an informal NATO meeting in Berlin at the weekend, according to the foreign ministry.