Stockholm and Helsinki submitted their bids to join NATO last week, reversing decades of military non-alignment, after political and public support for membership soared following Russia's invasion of Ukraine.

But Turkey, a NATO member, is throwing a spanner in the works as any membership must be unanimously approved by all members of the military alliance.

"Unlike many, I am optimistic... I am convinced that this disagreement will be resolved in the best way, in the spirit of NATO solidarity," Polish Foreign Minister Zbigniew Rau said at a joint press conference with his Romanian and Turkish counterparts.

"Sweden and Finland becoming members of NATO is vital to making us stronger," he said, though adding that their joining "should however benefit all NATO allies, including Turkey".

Romania's Foreign Minister Bogdan Aurescu said he supported "constructive dialogue".

"We hope to soon have good news concerning Sweden and Finland," he said.

Ankara accuses Stockholm in particular of providing a haven for the outlawed Kurdistan Workers' Party (PKK), listed as a "terrorist" group by Turkey and its Western allies.

It also accuses Sweden of harbouring supporters of Fethullah Gulen, the US-based preacher wanted over a failed 2016 coup in Turkey.

Turkey's Foreign Minister Mevlut Cavusoglu said his country had made "legitimate" demands of Sweden and Finland.

"These countries back terrorism. They must end this support and lift restrictions on defence industry sales to our country" imposed after the Turkish army's Syria operation in 2019, he said.