Ankara has accused the two Nordic nations of providing a safe haven for outlawed Kurdish militants it deems "terrorists" and held back on ratifying their NATO membership despite an agreement in June.

"President Erdogan noted that the steps to be taken by Sweden and Finland would determine how fast the approval process... would go and when it would be concluded," the Turkish presidency said.

Erdogan and Stoltenberg held a private meeting in Istanbul that was closed to the media.

Finland and Sweden dropped decades of military non-alignment and scrambled to become NATO members in May, after Russia invaded Ukraine.

But Erdogan threatened to block their bids and sought concessions, leading to a deal in June between Turkey, Finland and Sweden that included provisions on extraditions and sharing information.

New Swedish Prime Minister Ulf Kristersson will visit Ankara on Tuesday to meet with Erdogan in a trip that Stockholm hopes will lead to Turkey's approval.

Stoltenberg "welcomed the major, concrete steps already taken by both countries to put the memorandum into practice, and stressed that their accession will make NATO stronger", the alliance said in a statement on Friday.

On Thursday, the NATO secretary general said Finland and Sweden's accession was important "to send a clear message to Russia" during a press conference with the Turkish foreign minister.

All 30 NATO member states except Turkey and Hungary have ratified the accession of Sweden and Finland.

New members to the alliance require unanimous approval.