Torture, beating and sodomising inmates with brooms or pistol barrels were the norm at an illegal prison run by a military unit under the command of the Iraqi prime minister’s office, Human Rights Watch said.

The rights group on Wednesday called for a thorough investigation over the detention centre, which was discovered and closed down this month by Iraq’s Human Rights Ministry, and urged Iraq to prosecute those responsible.

Shi’ite Prime Minister Nuri al-Maliki has denied any connection with the facility, which housed mainly Sunni Arab prisoners from the volatile northern city of Mosul where insurgent groups such as al Qaeda operate.

The revelation of the prison came at a sensitive time for Maliki as he tries to negotiate alliances with other factions that would allow him to be reappointed as prime minister following an inconclusive election in March.

Sunni outrage at the reports of their compatriots being mistreated by majority Shi’ite-led authorities could increase sectarian tensions just as violence is beginning to recede.

Human Rights Watch interviewed 42 of the 300 men who had been detained on a military base at Baghdad’s old Muthanna airfield after being arrested in Mosul and accused of terrorism.

“The men’s stories were credible and consistent. Most of the 300 displayed fresh scars and injuries they said were a result of routine and systematic torture they had experienced at the hands of interrogators at Muthanna,” Human Rights Watch said.

The detainees said many were handcuffed, blindfolded and hung upside down. Interrogators kicked, whipped and beat them.

Interrogators also placed dirty plastic bags over their heads to close off air supply. When the detainees passed out, interrogators awakened them with electric shocks to the genitals or other parts of the body, Human Rights Watch said.

One detainee, a former Iraqi army general who had been living in London but returned to Mosul after his son was detained, said his jailors refused to give him medicine for his diabetes and high blood pressure, and beat him severely.

“They applied electricity to my penis and sodomized me with a stick,” the man, who is in a wheelchair, told Human Rights Watch. “I was forced to sign a confession that they wouldn’t let me read.”

Another detainee, who was 21, said interrogators threatened to rape his mother and sisters if he did not confess. During one torture session, guards made another detainee rape him.

Another detainee said he was sodomised with a pistol.

The Human Rights Ministry says three Iraqi army officers have been arrested for questioning. The prison was illegal because it was not under the jurisdiction of the Justice Ministry and the Human Rights Ministry was not informed of it.

Conditions in legal Iraqi prisons are often not much better. The justice system relies on confessions for prosecutions, not evidence. That makes torture common though perhaps not as routine as under ousted dictator Saddam Hussein.

“What happened at Muthanna is an example of the horrendous abuse Iraqi leaders say they want to leave behind,” said Joe Stork, deputy Middle East director at Human Rights Watch.

“Everyone responsible, from the top on down, needs to be held accountable.”

(Editing by Giles Elgood)