After Russia invaded Ukraine, Finland and Sweden in May dropped decades of military non-alignment and sought to join NATO.

That requires a consensus within the US-led defence alliance, but Turkey and Hungary have so far not ratified their membership.

Turkey has demanded the Nordic countries take a tougher stance on Kurdish groups it deems "terrorists" in exchange for its backing.

Mahmut Tat was sentenced to more than six years in jail over being a member of the PKK in Turkey. He fled to Sweden in 2015, but Stockholm rejected his asylum request.

Tat arrived in Istanbul on Friday night after Sweden detained and extradited him, the Anadolu news agency reported.

Turkish police arrested him soon after arriving at Istanbul airport and referred him to a court on Saturday, which sent him to jail, the news agency said.

Turkey has accused Finland and Sweden in particular of providing a safe haven for outlawed Kurdish groups it deems "terrorists", and held back on ratifying their NATO bids despite an agreement in Madrid in June.

Foreign Minister Mevlut Cavusoglu held trilateral talks with his Swedish and Finnish counterparts on the margins of a NATO meeting in Bucharest this week.

"The statements (coming out of Sweden) are good, the determination is good, but we need to see concrete steps," Cavusoglu said.

Ankara has said it expects Stockholm to take action on issues including the extradition of criminals and freezing of terror assets.