A United Nations court called on Turkey to free a judge caught up in the country's post-coup crackdown, saying Aydin Sefa Akay's imprisonment violates his diplomatic immunity and the principle of judicial independence.

Akay, both a UN judge and diplomat, is one of 40,000 Turkish officials who have been remanded in custody for alleged connections to July's failed military coup, blamed by authorities on followers of exiled cleric Fethullah Gulen.

In an order issued on Tuesday, UN court president Theodor Meron said Turkey should cease all proceedings against his fellow judge and free him by February 14, in time for him to assume his duties in a case involving a Rwandan genocide suspect.

"Diplomatic immunity is a cornerstone of an independent international judiciary," Meron wrote. Replacing Akay on the bench because of his detention would have a "chilling effect" on judicial independence by making judges seem easily replaceable.

According to Turkey's Hurriyet newspaper, Akay was arrested for having a messaging application on his mobile phone that was allegedly used by many of the plotters.

Turkey was a strong early backer of the international courts set up in the 1990s to try mass crimes from the Yugoslav wars and the Rwandan genocide, but it has taken a more unilaterally nationalist stance under President Recep Tayyip Erdogan.

Turkey has ignored summons to participate in UN court proceedings involving the judge.

Critics of Erdogan say he is using the post-coup dragnet as an excuse to purge political rivals. Gulen denies involvement in the coup, which claimed some 240 lives, and condemned it.

Akay had been due to hear pleadings from lawyers for Rwandan politician Augustin Ngirabatware, now serving 30 years for inciting genocide, who say they have turned up exonerating evidence that merits a review of his case.

Meron rejected their request for Ngirabatware to be given provisional release since such decisions could only be made by a full bench - something that is impossible while Akay remains imprisoned in Turkey.

Ngirabatware was a former planning minister who was convicted for direct and public incitement to commit genocide, and rape as a crime against humanity by the International Criminal Tribunal for Rwanda in 2012.

He is also the son-in-law of wanted businessman and alleged 1994 Genocide against the Tutsis mastermind Félicien Kabuga.

Ngirabatware's case remained with the UN Mechanism for the International Criminal Tribunals in The Hague.

The MICT is the legal successor to the courts that tried crimes committed during the Yugoslav wars and the Rwandan genocide.