Selahattin Demirtas, 51, was sentenced last May to 42 years in prison for his alleged role in deadly 2014 protests that erupted as Islamic State group jihadists overran the Syrian town of Kobane.

A two-time election rival to President Recep Tayyip Erdogan, he was convicted on dozens of charges including undermining state unity and the country's integrity.

But Erdogan has endorsed recent efforts by the DEM to establish a dialogue aimed at ending the conflict with the Kurdistan Workers' Party (PKK), which has waged an insurgency against the Turkish state since 1984, claiming tens of thousands of lives.

The DEM said it met with Demirtas, who announced his retirement from politics after Erdogan's 2023 re-election, at his prison in Edirne, western Turkey.

The visit, announced in a video, came after the party met in late December with Abdullah Ocalan, the jailed leader of the PKK who has been serving a life sentence on the island of Imrali south of Istanbul since 1999.

It was the first visit by the party in almost a decade and a clear sign of easing tensions between Ankara and the PKK, which is designated a terror group by Turkey and its Western allies.

The PKK has waged an insurgency against the Turkish state since 1984, claiming tens of thousands of lives.

But Ocalan said after the visit that he was "ready to make a call" to back the new peace initiative.

Demirtas had already said in October that "if Ocalan takes the initiative and wants to open a path to a [political] solution, we will support it with all our efforts".

Erdogan also reiterated his support for the process Saturday during a visit to Diyarbakir, the largest Kurdish city in Turkey, as fighting raged across the border in northeast Syria between Kurdish, Syrian and foreign combatants, including from the PKK, after the fall of strongman president Bashar al-Assad.

"A new window of opportunity has opened for our country to end the scourge of terrorism once and for all," Erdogan said.

"We are not going to miss the historic chance," he said. "Terrorist organisations have no other choice but to lay down their weapons, we are giving them the chance to become a true political organisation."

DEM lawmakers on Sunday are due to meet with the party's other co-president, Figen Yuksekdag, who was sentenced last May to 30 years in prison on charges of terrorist propaganda.