BAGHDAD, Iraq -- The highest-ranking soldier charged in the Abu Ghraib scandal pleaded guilty Wednesday to five charges of abusing Iraqi detainees at the prison as a two-day court-martial opened at a U.S. base in Baghdad.

U.S. Army reservist Staff Sgt. Ivan "Chip" Frederick, 38, of Buckingham, Va., admitted to allegations of conspiracy, dereliction of duty, maltreatment of detainees, assault, and committing an indecent act. He was expected to be sentenced Thursday.

Under a plea bargain deal, several other charges against Frederick -- a military policeman and a corrections officer in civilian life -- were dropped, according to his attorney Gary Myers.

The Abu Ghraib prisoner scandal broke in April with the worldwide publication of photos and videos showing American soldiers abusing and humiliating naked Iraqi detainees.

Frederick is alleged to have watched as a group of detainees were made to masturbate while other soldiers photographed them. He also is accused of jumping on a pile of detainees, stomping on detainees' hands and bare feet, and punching one in the chest so hard he needed medical attention.

In addition, Frederick allegedly helped place wires in a detainees' hands and told him he would be electrocuted if he fell off a box.

Frederick is one of seven members of the Cresaptown, Md.-based 372nd Military Police Company charged in the scandal. One, Spc. Jeremy C. Sivits, of Hyndman, Penn., is serving a one-year prison sentence after pleading guilty in May to three counts.

Spc. Armin Cruz, 24, a Military Intelligence soldier, was sentenced last month to eight months confinement, reduction in rank to private and a bad conduct discharge for his part in the scandal.

The proceeding includes witness testimony and other evidence but almost all the witnesses will participate by video teleconference from outside Iraq, Frederick's lawyer said.

Several of the defense witnesses will be in Washington or Europe. One of the government's two witnesses also will testify remotely, from Mannheim, Germany.

The unusual arrangement ensured better cooperation from witnesses who were afraid to visit a war zone after a military appellate court refused to move the proceeding out of Iraq, according to Myers.

Frederick's wife, Martha, is on Myers' witness list. "I don't think anybody in their right mind would be willing to go to Iraq," she said.