Turkish President Recep Tayyip Erdogan said on Monday he would only back Sweden's Nato candidacy if the European Union resumes long-stalled membership talks with Ankara.

The mercurial Turkish leader's unexpected new demand came on the eve of a landmark Nato summit at which Western leaders want to display unity in the face of Russia's war on Ukraine.

Erdogan has been blocking Sweden's accession for months because of what he deemed was Stockholm's failure to crack down on Kurdish militants who have been waging a bloody insurgency against the Turkish state.

His new condition -- never raised in public before -- threatened to open a new chasm between Turkey and its Western allies in the heat of Europe's worst conflict since World War II.

Germany's powerful Chancellor Olaf Scholz quickly called Erdogan's demand unrelated to the issue of giving Sweden the security guarantees afforded by the world's most powerful military organisation.

Erdogan argued on Monday that many of the same leaders who were blocking Turkey's EU membership now wanted him to support Sweden's candidacy.

"Almost all the Nato members are EU members. I now am addressing these countries, which are making Turkey wait for more than 50 years, and I will address them again in Vilnius," Erdogan said moments before boarding his flight for the summit in the Lithuanian capital.

"First, open the way to Turkey's membership of the European Union, and then we will open it for Sweden, just as we had opened it for Finland."

Erdogan added that "this is what I told" US President Joe Biden when the two leaders spoke by phone on Sunday.

Turkey first applied to be a member of the European Economic Community -- a predecessor to the EU -- in 1987.

It became an EU candidate country in 1999 and formally launched membership negotiations with the bloc in 2005.

The talks stalled over European concerns about human rights violations that came in the midst of a sweeping crackdown Erdogan launched after surviving a failed 2016 military coup.

Scholz said Erdogan's new demand conflated two unrelated issues.

"Sweden meets all the requirements for Nato membership," he said in Berlin. "The other question is one that is not connected with it and that is why I do not think it should be seen as a connected issue."

Erdogan had previously voiced repeated frustrations with what he calls Sweden's failure to keep its promise to deal with suspected Kurdish militants allegedly "roaming the streets" of Stockholm.

The dispute stemmed from a deal Turkey signed with Sweden and Finland after the neighbours ended decades of military non-alignment and sought shelter under the nuclear umbrella afforded by Nato after Russia invaded Ukraine.

An agreement struck at last year's Nato summit committed Sweden and Finland to toughen their anti-terrorism legislation and hand suspected militants to Turkey.

Erdogan harboured far fewer grievances against Finland and Helsinki was formally welcomed into the alliance on April 4.

The Turkish leader barely mentioned the dispute with Sweden on Monday.

"We want the promises given to us to be kept and our determination on this is the same," he said.

Sweden has already toughened up its anti-terror legislation and lifted an arms embargo it had imposed on Turkey after Erdogan launched a unilateral incursion into northern Syria in 2019.

The Swedish government last month also agreed to extradite a man convicted of drug trafficking who had supported the pro-Kurdish PKK militia -- a first that Nato chief Jens Stoltenberg played up in public remarks last week. (AFP)