Sweden's bid for NATO membership is facing a dead end as ties have recently grown increasingly strained with Turkey.

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

"We can now probably forget Turkish ratification before the elections, which seem to be scheduled for May 14," Levin told AFP, adding that "what happens after that depends in part on who wins."

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

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, which is listed as a "terror group" by Turkey.

"There are groups in Sweden who are against NATO membership and PKK supporters worried about the government's pledges to go after" them, Levin said, adding that these supporters had realized that they could provoke Erdogan "by insulting him and thereby stall the accession process."

Earlier this month, Ankara summoned Sweden's Ambassador after pro-Kurdish activists hung an effigy of Erdogan from its feet.

Sweden's Prime Minister Ulf Kristersson denounced it as an attempt to sabotage the country's NATO membership bid.

In the same context, Turkish Defense Minister Hulusi Akar said on Saturday that Swedish Defense Minister Pal Johnson's visit to Ankara has been called off following Sweden's failure to respond to Turkey's demands, which involved taking measures against the PKK and addressing Stockholm's green card to the leader of the far-right Danish political party Stram Kurs to burn a copy of the Holy Quran.

Last week, the leader of the far-right Sweden Democrats, Jimmie Akesson, denounced Erdogan as an "Islamist dictator". 

Akesson urged Kristersson not to appease Turkey "because it is ultimately an anti-democratic system and a dictator we are dealing with," he told the Swedish daily Dagens Nyheter.

Turkey is seeking the extradition of more PKK members based in Sweden. Erdogan recently said there were as many as 130 there.

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

NATO Secretary-General Jens Stoltenberg, 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, Stoltenberg noted.

Read more: Erdogan warns of 'new low' in relations with Finland, Sweden