Prime Minister Ulf Kristersson and NATO Secretary General Jens Stoltenberg (Photo: NATO)

Until Finland and Sweden take the necessary "steps," Turkey will not ratify their membership of NATO, Turkish President Recep Tayyip Erdogan told the alliance's chief Jens Stoltenberg Friday.

The two Nordic nations have been accused by Turkey 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.

The Turkish President and the NATO chief held a private meeting in Istanbul that was closed to the press.

Finland and Sweden dropped decades of military non-alignment and scrambled to become NATO members in May after the start of the Ukraine war.

Earlier, on October 1st, Erdogan said that Turkey will continue to freeze Sweden and Finland's bids for NATO membership until the promises made by the two Nordic countries are "kept".

"Until the promises made to our country are kept, we will maintain our principled position," Erdogan said in a speech to parliament in Ankara.

New Swedish Prime Minister Ulf Kristersson will head to Turkey on Tuesday to see Erdogan in a visit that Stockholm hopes will result in Turkey's approval.

Kristersson pointed out that he aimed to show Turkish President Recep Tayyip Erdogan that Sweden and Finland "actually do what we promised" to fulfill a deal with Ankara to clear their path into NATO.

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.

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

Twenty-eight of NATO's 30 allies have so far ratified the accession of Finland and Sweden, leaving only Turkey and Hungary to sign off before they officially join the group.