Nato secretary general Jens Stoltenberg on Sunday called on Ankara to drop its opposition to Sweden's bid to join the US-led defence alliance, hoping Stockholm's accession would be finalised "as soon as possible".

Pressure is building on Erdogan to greenlight Sweden's Nato membership ahead of a summit planned for July in the Lithuanian capital Vilnius.

"Membership will make Sweden safer but also make Nato and Turkey stronger," Stoltenberg told journalists after meeting with President Recep Tayyip Erdogan and newly appointed foreign minister Hakan Fidan, who was head of the intelligence agency, in Istanbul.

"I look forward to finalising Sweden's accession as soon as possible," he said.

On Saturday, Stoltenberg attended the inauguration of Erdogan, who was re-elected to serve another five years, in a lavish ceremony joined by dozens of world leaders in the capital Ankara.

Nato member Turkey has dragged its feet over admitting Sweden to the military alliance. It and Hungary are the only two Nato countries yet to ratify the membership bid.

Sweden and Finland dropped decades of military non-alignment and applied to join the alliance in response to Russia's war with Ukraine.

Finland formally joined Nato in April.

Erdogan has accused Sweden of being a haven for "terrorists", especially members of the Kurdistan Workers' Party (PKK), a group blacklisted by Turkey and its Western allies.

"Sweden has taken significant concrete steps to meet Turkey's concerns," Stoltenberg said.

"This includes amending the Swedish constitution, ending arms embargo, stepping up counter-terrorism operations including against the PKK," he said.

"Sweden has fulfilled its obligations."

Swedish Foreign Minister Tobias Billstrom also said Stockholm had fulfilled "all the commitments" to join Nato and urged Turkey and Hungary to allow his country into the alliance. (AFP)