NATO Secretary General Jens Stoltenberg met representatives from the three countries to try to make progress on the Nordic nations' membership applications, which have been blocked by Ankara.

While he described the talks as "constructive" Turkey made it clear there was still work to be done.

"The Madrid NATO summit is not the deadline, so our negotiations will continue," Turkish President Recep Tayyip Erdogan's foreign policy adviser Ibrahim Kalin told reporters after the talks.

Even before this latest meeting some observers were playing down the chances of a deal at the summit.

"I think it is possible but it would be very difficult," Paul Levin, director of the Institute for Turkish Studies at Stockholm University, told AFP, adding it would require compromise on both sides.

NATO, Stockholm and Helsinki had expected the application process to be quick. But Ankara's objections caught them all off guard, at a time when NATO is keen to display a unified front against Russia.

Ankara has accused Finland and Sweden of providing a safe haven for the Kurdistan Workers' Party (PKK), listed as a "terrorist" group by Turkey and its Western allies.

Ankara has also demanded they lift their weapons freezes on Turkey.

- Kurdish quandary -

Any NATO membership deal must be unanimously approved by the 30 members of the alliance, and fears are now mounting that Turkey could delay the Nordic countries' bids indefinitely.

Finnish Prime Minister Sanna Marin recently expressed fears that unless the issues are resolved before the NATO summit in Madrid, "there is a risk that the situation will freeze".

Germany also dampened hopes of a deal being reached that quickly.

"I think this is about expectations management and to place this in its historical context," said a high-ranking German government source Monday, while stressing a solution was still in sight.

"It would not be a catastrophe if we need a few more weeks," the source said. "What is crucial is that in our view there are no insurmountable difficulties" between Sweden, Finland and Turkey.

Speaking in Luxembourg on Monday, Sweden's Foreign Minister Ann Linde told Swedish media that "we should be prepared for this to take some time".

Ankara's anger has primarily been directed at Sweden.

"Sweden does view the PKK as a terrorist organisation and has done so since 1984," Levin said, adding that it was "arguably the first country apart from Turkey" to do so.

"So in that sense Sweden does not really stand out" from other European countries.

Sweden has however expressed support for the YPG, a US-backed Syrian Kurdish group, and its political arm, the Democratic Union Party (PYD).

Ankara views the YPG, which fought against the Islamic State group in Syria with Western support, as the PKK's Syria offshoot.

In a bid to ease Ankara's concerns, Swedish Prime Minister Magdalena Andersson has stressed that Sweden has been beefing up its anti-terror laws in recent years, with new stricter legislation coming into force on July 1.

Sweden has also said that its independent weapons export agency would be prepared to review its policy once the country was a NATO member.

Levin noted that one area where Sweden, which is home to around 100,000 Kurds, does stand out in Europe is that it is "generally more sympathetic to the broader Kurdish cause".

"In that sense, maybe Turkey is right to put the spotlight on Sweden", Levin said.

- Sweden's hands tied -

Sweden's government is also being squeezed on the home front, with its hands tied by an independent lawmaker with Kurdish roots.

Amineh Kakabaveh is a former Left Party member of Iranian-Kurdish origin sitting in parliament as an independent since 2019.

In November, she provided the deciding vote to bring the Social Democrats into power -- in exchange for deeper cooperation with the PYD.

Kakabaveh has threatened to vote against the government's budget proposal this week if Sweden agrees to sell arms to Turkey.

But Sweden's legislative elections in September could end the deadlock with Ankara.

Kakabaveh is not expected to be re-elected to parliament, which would enable the government to negotiate more freely with Turkey.

"It really looks like the Swedish government is trying to step away from this agreement with Kakabaveh in order to be able to have this discussion with Turkey," Levin said.