Turkey Wednesday put on trial a top UN court judge on charges of links to the group blamed for the failed July 15 coup, in a case that has caused anger and held up legal proceedings over the Rwanda genocide.

Aydin Sefa Akay, a top judge attached to the UN's Mechanism for International Criminal Tribunals (MICT), was detained in September at the family home and had been held in detention since.

He is charged with "membership of a terror group" over alleged links to the organisation of Fethullah Gulen, the US-based preacher blamed by Ankara for the July 15 coup bid.

Specifically, he stands accused by the Turkish authorities of downloading and using a messaging app called ByLock which Ankara says was used by the plotters to prepare the coup.

Akay and his lawyers took part in the first hearing at the Ankara criminal court, the state-run Anadolu news agency said.

If found guilty, he faces up to 15 years in jail.

In his statement, Akay denied the charges, saying he was not a member of Gulen's group. "I am not one of them," he said.

According to Anadolu, he admitted downloading Bylock but said he had not used any password to access the system.

His lawyers called for his release, arguing Akay should enjoy immunity due to his status. But the court ordered he be kept under arrest, setting the next hearing for April 13.

Akay had been working with the UN international court trying suspects over the 1994 genocide in Rwanda and his detention has paralysed proceedings into an appeal hearing of former Rwandan minister Augustin Ngirabatware.

The UN court has already said Turkey has failed to comply with its obligations and said it would report Ankara to the UN Security Council.

Presiding judge Theodore Meron has insisted that Akay, a former diplomat who was nominated to the bench by Ankara, has diplomatic immunity and has repeatedly voiced concern over the conditions of his detention.

Under a state of emergency imposed after the coup, Turkey has embarked on a relentless crackdown against alleged supporters of Gulen, arresting some 43,000 people.