The first trials of the thousands of suspects arrested in the wake of Turkey's failed July 15 coup aimed at ousting President Recep Tayyip Erdogan will begin in early 2017, the chief Ankara prosecutor said on Thursday.

Thousands of ex-soldiers, legal workers and civilians are currently in jail pending trial on suspicion of involvement in the coup, which Ankara says was masterminded by the US-based Islamic preacher Fethullah Gulen.

"Without giving an exact date, we are expecting to begin the first trials at the start of 2017," Ankara's chief prosecutor Harun Kodalak said in comments published by Turkish media.

He added: "There could be a trial or trials opened in the last month of this year but it is in 2017 that we are planning to open a series of trials," he said.

The trials are expected to be the most substantial legal process in Turkey in its modern history, with purpose-built facilities needed to be set up in some areas.

The scale of the crackdown and the duration of the suspects' stay in pre-trial detention has caused international concern and strained Turkey's ties with the European Union.

Gulen, who has been based in the United States since the late 1990s, has vehemently denied the claims of his involvement in the coup.

Turkish officials have in recent days said two civilians -- a theology lecturer named Adil Oksuz and businessman Kemal Batmaz -- were in charge of organising the coup bid from the Akinci airbase in Ankara.

According to prosecutors, the two were in the United States and only returned to Turkey two days before the coup bid.

Batmaz is in custody in Sincan prison outside Ankara. Embarrassingly for the Turkish authorities, Oksuz was detained in the aftermath of the coup but then released and is now on the run.

According to Kodalak, video footage has confirmed that Batmaz was at the base on the night of the coup. Batmaz has denied involvement in the coup.

Turkish officials have said that Oksuz was the so-called "imam" of the plot and in charge of coordinating between Gulen and the army.

But Kodalak said Batmaz "could be as important as Adil Oksuz and even his superior".

According to a report Wednesday by Turkey's NTV television, the authorities had thwarted a mass escape plan by 5,544 coup suspects.