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.

"I think it's very important to go forward at this stage. If we don't solve these issues before Madrid, there is a risk that the situation will freeze," Prime Minister Sanna Marin said, referring to the upcoming NATO summit in Madrid, scheduled to begin on June 28.

"We don't know for how long but it might freeze for a while," Marin told reporters during a visit to Sweden.

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.

Marin stressed that they were taking Turkey's concerns seriously and wanted to address them "and if there are some misunderstandings, to correct those".

At the same time she also noted that Turkey had previously said the countries would be welcomed into NATO, but that Ankara changed its tune once the membership applications were made.

"Of course we are taking all the issues very seriously and are having talks, but I also think that it's also Turkey's responsibility to try to find solutions at this stage," Marin said at a joint press conference with her Nordic counterparts.

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".

Stoltenberg has previously said Sweden and Finland would be welcomed into NATO "with open arms", and expected the Turkey issue to be resolved before the upcoming NATO summit.

But speaking in Finland on Sunday he conceded that the dispute may drag on, saying "the summit in Madrid was never a deadline".