Former Turkish army chief Ilker Basbug, the highest-profile defendant convicted in a mass trial over an alleged coup plot, was set to walk free from jail on Friday, less than a year after being given a life sentence.

A staunch defender of the military in its showdown with the Islamic-rooted government of Prime Minister Recep Tayyip Erdogan, Basbug branded his trial a "black stain" on the country's history.

The 71-year-old, a former NATO-trained career soldier seen as a political moderate, once led Turkey's military campaign against outlawed Kurdish rebels but later took a conciliatory approach to resolving the three-decade conflict.

The decorated general was sentenced to life in prison last August after a long-running trial of 275 people accused of plotting to overthrow Erdogan's Justice and Development Party (AKP) government.

But on appeal, Turkey's top court ordered his immediate release, ruling that Basbug's rights were violated because a lower court had failed to publish a detailed verdict and send it to the appeals court.

Perhaps not coincidentally, the order for his immediate release came as an embattled Erdogan is seeking to improve ties with the military as he fights for his political survival in a feud with Muslim cleric Fethullah Gulen.

University professor Ahmet Insel, author of two books on the Turkish armed forces, said he does not count Basbug among the hardline generals who in 1997 helped bring down the government of the Islamist premier Necmettin Erbakan, Erdogan's mentor.

"He is a general who tries to keep the Turkish army in the barracks," Insel said, in a country where the military has carried out three coups since 1960.

He said Basbug was known to lean towards the main opposition Republican People's Party (CHP), although he has never voiced public support for the party created by Mustafa Kemal Ataturk, the founder of modern Turkey and the embodiment of the secular values the army defends.

- 'My world turned black' -

Basbug, who led the army from 2008 to 2010, has written two books about Ataturk, describing him as the "greatest leader of the 20th century".

As early as 2007, Basbug spoke out in defence of the first army officers to be implicated in the alleged coup plot, a group he would later find himself joining in the dock.

"He gave the impression of someone ready to say anything to defend the army," said Insel.

Despite being in charge of the military campaign against Kurdish rebels, observers say Basbug is no hardliner and in fact advocated a moderate approach towards the outlawed Kurdistan Workers' Party (PKK) during his tenure as military chief.

"He's no hawk," Sinan Ulgen, head of the Istanbul Centre for Economics and Foreign Policy Studies, told AFP in an interview.

Born in Afyonkarahisar in western Turkey in April 1943, he served in various Turkish army units and held a number of command positions.

He studied at Britain's Army Staff College and NATO Defence College and served at NATO'S Supreme Headquarters Allied Powers Europe (SHAPE), according to a NATO biography.

He became a full general in 2002, and was head of the army before taking overall command in August 2008 of Turkey's 515,000-strong military, a force second only to the United States in the NATO alliance.

He held the post until his retirement two years later and has since written four books, including one on terrorism.

Basbug, who was arrested in January 2012, also penned a book while in jail, balking at the charges against him.

"Leading and running a terrorist organisation! Let me be clear, my world turned black after I heard the charges," he wrote.

He also expressed resentment about a lack of loyalty and support from his comrades.

"The commander of such an army facing charges of forming and leading an armed organisation is really tragicomic," he said in his initial testimony. "I always followed the law and the constitution throughout my tenure."

Observers say Basbug's tenure as military chief coincided with a perceived cooling of the Kurdish conflict, which flared up again when he left office.

"He has made very constructive comments on the Kurdish question," Ulgen said.

In 2009, for example, Basbug urged Ankara to make sure that its Kurdish citizens benefit from "equal opportunities" and to strive to change the perception among Kurds that "they are being victimised".

Basbug is married and has two children.