The Indonesian army admitted on Thursday that its special forces stormed a jail and shot dead four prisoners who were awaiting trial accused of killing their superior officer.

Nine members of the Special Forces Command confessed to carrying out the revenge killings shortly after midnight on March 23 at a central Java jail, said Brigadier General Unggul Yudhoyono, who led a probe into the killings.

The prisoners were accused of beating and stabbing to death the soldiers' superior officer, Heru Santoso, at a nightclub in Yogyakarta, central Java, several days earlier as he reportedly tried to break up a fight.

Rights groups hailed the admission of guilt, as the army has rarely made such confessions despite being regularly accused of the killing and abuse of civilians, particularly in restive eastern Papua.

The military probe "deserves applause but also comes as a surprise because in the history of the military such findings are rare," said the head of the Setara Institute of Peace and Democracy, Hendardi, who goes by one name.

However Yudhoyono also stressed that the attack in Sleman district was spontaneous and to defend the "dignity of the forces after hearing of the sadistic killing by thugs in Yogyakarta" of Santoso.

He also noted that "the perpetrators have gallantly admitted their deed on the first day of our investigation". Two of the soldiers' colleagues tried to stop them from carrying out the attack, he said.

Yudhoyono gave details of the mens' confessions as he announced the results of a preliminary investigation. He said a full probe would now be held and the soldiers would face a military court.

The army had insisted on conducting an internal investigation despite requests from the national human rights commission that it be allowed to probe the killings.

Indonesia's special forces were frequently accused of killing civilians in Papua, East Timor and Aceh under the rule of authoritarian former president Suharto, who stepped down in 1998.