Stoltenberg is due to meet with President Recep Tayyip Erdogan, who has warned his country will not give a nod to the two countries' memberships until "the promises they made were kept".

Erdogan has accused Finland and Sweden in particular of providing shelter to outlawed Kurdish militants deemed "terrorists" by Ankara.

In June, Turkey, Sweden and Finland struck a deal which included provisions on extraditions and sharing of information.

The two Nordic nations earlier this year ditched their longstanding policies of non-alignment, asking to join NATO because of Russia's invasion of Ukraine and how it reshaped Europe's security.

Stoltenberg on Wednesday announced plans to visit Turkey while praising the "close contact" Stockholm and Helsinki now had with Ankara "at all levels".

He said: "I will go to... Istanbul to meet with President Erdogan in the near future myself."

Erdogan has also accepted a request from Sweden's new Prime Minister Ulf Kristersson to visit Turkey as Stockholm struggles to overcome Ankara's blocking of its NATO bid.

The Turkish official, who wished to remain anonymous, said the visit was likely to take place on November 8.

A spokeswoman for Kristersson could not confirm the date. "We're working on it," she said.

Erdogan said during a telephone call with the Swedish premier this week that Turkey "stands ready to advance the bilateral relations with the Swedish government in all areas".

NATO accession is a priority for Sweden's new right-wing government.