"My testimony is invalid. I signed it without knowing the content," Firat Altin, a former Kurdish militant, told the court in Diyarbakir, adding that he had faced "threats and blackmail" by judicial officials.

Another witness, Hidir Altug, said he testified under pressure from a former police chief and was made to memorise the allegations he made.

The indictment says Colonel Cemal Temizoz set up a rogue unit that detained and questioned people for helping Kurdish rebels or to settle personal scores when he served in southeastern Sirnak province from 1993-1996.

Acting on orders from Temizoz, the unit allegedly killed some of those questioned, often burying the bodies in uninhabited areas.

The six other defendants include former militants of the Kurdistan Workers' Party (PKK) who became informers and collaborated with the military, and members of the "village guard" -- a local Kurdish militia armed and paid by the government to help the army against the PKK.

All seven suspects risk life sentences with no chance of parole.

Both Altin and Altug, former PKK militants who turned informers years ago, are among the seven defendants, but they disclosed Friday that the prosecution had used them also as secret witnesses and given them code names in the indictement.

"I am neither a secret nor open witness," Altug said after he retracted his testimony.

Altin said a prosecutor had told him he should be now loyal "not to the old state but to us."

Army officers were recently targeted in several controversial court cases, which government opponents say have been tailored by associates of the ruling Islamist-rooted Justice and Development Party in a bid to discredit the influential, staunchly secularist military.

The judge set the next hearing for October 9.

The opening of the trial last week was hailed as a major step in shedding light on widespread allegations that the security forces acted outside the law in their struggle against the PKK in the 1990s when the group's armed campaign was at its peak.

Temizoz rejected the charges, saying the murders were committed by PKK rebels themselves amid in-fighting within the group, or as part of personal setting of accounts and blood feuds, which are frequent in the Kurdish-majority southeast.

The indictment followed excavations in the region in March and April that resulted in the discovery of bone fragments.

The probe began after a former PKK member who later became an informer told the Turkish media that said several people who disappeared in the 1990s were executed summarily.

The PKK, listed as a terrorist group by Ankara and much of the international community, took up arms in 1984, sparking a conflict that has claimed some 45,000 lives.