ISTANBUL - A Turkish court issued arrest warrants Monday for eight people, including the alleged mastermind of the 2016 failed coup, Fethullah Gulen, over the murder in Ankara that year of the Russian ambassador.

The warrants were issued a day ahead of a visit to Ankara by President Vladimir Putin for talks with Turkish counterpart Recep Tayyip Erdogan aimed at bolstering the growing Moscow-Ankara relationship. Ambassador Andrei Karlov was shot dead in December 2016 by off-duty Turkish policeman Mevlut Mert Altintas, 22, while inaugurating a photo exhibition.

Turkey has said the murder was a plot carried out by the group of Gulen, a US-based Islamic preacher who stands accused of staging the 2016 coup bid. But Moscow has yet to echo this.

According to the warrants, the eight suspects are charged with seeking to overthrow the constitution and pre-meditated murder, the state-run Anadolu news agency reported.

The other prominent figure among the eight is Serif Ali Tekalan who headed a university linked to Gulen in Istanbul and now heads the Texas-based North American University (NAU). It was not immediately clear if any of the suspects are currently in Turkey.

Gulen, who denies any link to the coup, has been hit with multiple Turkish arrest warrants. But so far the United States has yet to budge in response to Turkish demands for his extradition.

Anadolu said that the investigation had found links between the gunman Altintas and Gulen.

According to the news agency, nine people are already in custody awaiting trial over the killing including television producer Hayreddin Aydinbas, the organiser of the photo exhibition Mustafa Timur Ozkan and former police officers. There were fears the killing would again derail relations that had only been put back on track that summer with a reconciliation deal after the November 2015 shooting down by Turkey of a Russian war plane over the Syrian border. But relations have since gone from strength to strength, with Putin's upcoming visit expected to signal the strategic importance of the relationship to both sides.