Erdogan said fight against terror was one of the leading topics during his discussions with outgoing Dutch Prime Minister Mark Rutte, who was in Istanbul to rally Turkey's support for his candidacy.

"We told him that in accordance with the alliance spirit, no terror organisation, especially the PKK and its affiliates should be tolerated," Erdogan told reporters, alongside Rutte.

The PKK, listed as a terror group by Turkey and its Western allies, has waged a decades-long insurgency for greater autonomy for the Kurdish minority in the southeast of the country.

In addition, Erdogan said it is "imperative for the new secretary general to make intense efforts to remove sanctions, restrictions and obstacles in defence industry area among the allies".

Romania's President Klaus Iohannis is also a candidate for the NATO post, and Erdogan said he conveyed Turkey's requests to him during a phone call last week.

Erdogan did not reveal Turkey's choice between the two men, but said the decision would be based on "reason", even as he smilingly wished Rutte success on his "new journey".

Rutte is the frontrunner to replace Norwegian Jens Stoltenberg at the helm of the alliance after the United States, Britain and Germany expressed their support for his candidacy.

Hungary, however, said the country would not support Rutte, who has previously voiced concern about receding democratic standards under Prime Minister Viktor Orban.

Rutte is also facing scepticism over the Netherland's failure to hit NATO's defence spending targets over the past decade.