Sweden's bid for NATO membership is facing a dead end because Ankara's demands to Stockholm to hand over Kurdish activists and prevent rallies attacking Turkey's leadership have strained ties.

The chances of this changing after Turkey's parliamentary elections due in mid-May are uncertain, said Paul Levin, director of Stockholm University's Institute for Turkish Studies.

"We can now probably forget Turkish ratification before the elections, which seem to be scheduled for May 14," Levin told AFP.

"What happens after that depends in part on who wins."

President Recep Tayyip Erdogan's objections to Sweden's NATO membership rest largely on Stockholm's refusal to extradite Turkish nationals Ankara wants to prosecute for "terrorism".

And Erdogan is running for re-election.

In December, Sweden extradited a member of the Kurdistan Workers' Party (PKK) to Turkey. He had been convicted in a Turkish court and denied asylum in Sweden.

Erdogan wants more action from Stockholm against the PKK, listed as a terror group by Turkey and its Western allies.

"Tthere are groups in Sweden who are against NATO membership and PKK supporters worried about the government's pledges to go after them,” Levin said.

He said these PKK supporters had realised they could provoke Erdogan "by insulting him and thereby stall the accession process".

- Protests in Sweden -

A protest by a far-right politician in front of the Turkish embassy in Stockholm on Saturday -- authorised by the police -- has further strained relations.

Rasmus Paludan is a Swedish-Danish activist who has already been convicted for racist abuse.

He provoked rioting in Sweden last year when he went on a tour of the country and publicly burned copies of the Holy Koran.

On Saturday, he burned another copy of the Muslim Holy book after a hatred speech of almost an hour.

Ankara summoned Sweden's ambassador to register its outrage, then cancelled a visit of Swedish Defence Minister Pal Jonson that had been scheduled for next Friday in Ankara -- a rare high-level meeting.

Earlier this month, Ankara called in Sweden's ambassador after pro-Kurdish activists hung an effigy of Erdogan from its feet, explicitly comparing him to Benito Mussolini.

Turkey is seeking the extradition of more Kurdish "terrorists" based in Sweden. Erdogan recently said there were as many as 130 there.

Stockholm has made it clear that the courts have the final say, but that does not appear to have satisfied Ankara.

Jens Stoltenberg, the secretary general of NATO, who last spring was talking of a fast-track membership process of just a few weeks, told AFP in January he still thought it would happen in 2023, even if he could not guarantee it.

Both Turkey and Hungary were still to ratify the bid, he pointed out.

Both countries have maintained links with Russia since its invasion of Ukraine, with Ankara in particular adopting the role of mediator between the two sides.

One spark of hope for Sweden is that Finland, which also launched its bid to join NATO following the Russian invasion, has made it clear that it does not want to enter the alliance without its "big brother".