A court in Istanbul ruled on Thursday that a Turkish employee of the U.S. Consulate should remain in custody pending the outcome of his trial on charges of espionage and attempting to overthrow the Turkish government, the state-run Anadolu Agency reported.

The court’s interim decision could further strain ties between the two NATO allies. Metin Topuz, who has been held in pre-trial detention for nearly a year and a half, was expected to be released Thursday.

Topuz, 59, a translator and assistant for the U.S. Drug Enforcement Agency, is accused of links to U.S.-based Turkish cleric Fethullah Gulen, whom the Turkish government blames for the 2016 coup attempt. He faces a life sentence if convicted.

Topuz told the court in the opening hearing on Tuesday that he “never had any relations with the treacherous group,” in reference to Gulen’s network, which Turkey considers a terror group.

The trial was adjourned until May 15.

Topuz’s arrest in October 2017 led to the suspension of bilateral visa services between the U.S. and Turkey for more than two months and is one of several contentious issues that have increased tensions between Ankara and Washington.

A top U.S. official in February said the diplomatic mission’s “single most important priority” was securing the freedom of “our unjustly detained employees.”

Mete Canturk, another Turkish employee of the consulate, is under house arrest in a related investigation. Hamza Ulucay, a Turkish translator of the U.S. mission in southern Adana province was convicted of terror-related offenses earlier this year but released from prison.