A former US soldier, Megan Ambuhl, who was dismissed from the army last year after pleading guilty for her role in the prisoner abuses at the US-run prison near Baghdad, told the court: “They wanted me to be in the shower, point to the detainees’ genitals and laugh.”

This happened “fairly often” and involved other female MPs, she said during testimony at the Fort Hood army base in Texas.

Ambuhl was called to the witness stand by Specialist Charles Graner’s lawyers, who have maintained their client was merely following orders.

Ambuhl said that in one case, two interrogators encouraged Graner and another MP to “go in and rough up the detainee,” but she admitted the two were lower in rank than Graner, who was a corporal at the time.

Another soldier mentioned an incident in which prisoners were roughed up, saying low-ranking interrogators “called the shots” and Graner followed their lead.

But under cross-examination, the two witnesses admitted they had never heard any orders to pile up naked detainees, force them to masturbate or put them on a leash — acts which Graner is accused of conducting.

Graner himself did not take the stand, but could still do so before sentencing if the 10-man jury finds him guilty of the charges that carry a maximum penalty of 17 and a half years’ imprisonment, dishonourable discharge and forfeiture of pay.

Three Abu Ghraib inmates whose videotaped depositions were shown at the court-martial said that Graner subjected them to brutal beatings, sexual humiliation and religious insults.

Closing arguments were due on Friday, after which the jury will deliver its verdict.

Graner is the first soldier in the Abu Ghraib case to plead innocent to the charges of prisoner abuses.

The scandal surfaced last year after the publication of photographs of Iraqi prisoners being abused by US soldiers at the notorious prison.