The trip to meet President Recep Tayyip Erdogan will be a sensitive one, as Turkey is, along with Hungary, the last of NATO's 30 countries to ratify the accession protocol that would make Finland and Sweden new members.

The process needs to be unanimous. But Erdogan in early October warned his country would not ratify the two countries' memberships until "the promises" they made were kept.

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 reshuffled Europe's security.

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

It addressed Erdogan's main demands that Finland and Sweden cease hosting Kurdish militants outlawed in Turkey which he regards as "terrorists", and hand over Kurds wanted by Ankara.

Stoltenberg, speaking at a media conference after welcoming Romania's prime minister to NATO headquarters, hailed 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".

It would follow up on a visit new Swedish Prime Minister Ulf Kristersson is to make in the coming days.

Stoltenberg also said Hungary "has made it clear" that its parliament would vote on ratification of the accession protocols for Finland and Sweden in the next month or so.

"I'm confident that all allies will ratify the accession protocol," he said.

The NATO chief added, in a veiled warning to Russia, that security guarantees extended to Finland and Sweden pending their membership process continued to apply.

"If there were any kind of pressure against Finland and Sweden, it is inconceivable that the NATO allies should just stand idly by and not react. So we will react if there is any kind of pressure against Finland and Sweden," he said.