"We have transmitted our questions in writing to these two countries," Turkey Foreign Minister Mevlut Cavusoglu told reporters.

"Now we're waiting for their written responses," he said.

The two Nordic countries reversed decades of military non-alignment by applying for NATO memberships in May, following Russia's invasion of Ukraine.

Any NATO membership deal must, however, be unanimously approved by all 30 members of the alliance, and Turkey has thrown a spanner in the works and blocked their bids.

Ankara is accusing the Nordic neighbours of providing a safe haven for the outlawed Kurdistan Workers' Party (PKK), listed as a "terrorist" group by Turkey and its Western allies.

Turkey President Recep Tayyip Erdogan told members of his party on Wednesday that "as long as Sweden and Finland don't adopt concrete measures on the fight against terrorism, our position will not change."

NATO Secretary General Jens Stoltenberg said Monday, during a visit to Sweden, that NATO was working "hard and actively" to resolve Turkey's concerns "as soon as possible".

Finland's Prime Minister Sanna Marin acknowledged on Tuesday that the Nordic bids could stall if agreement with Turkey is not reached before a summit later in June.