JOINT BASE LEWIS-MCCHORD, Washington — An Army staff sergeant accused of leading a rogue “kill squad” charged with murdering three Afghan civilians took the stand Friday during his court martial and denied carrying out the killings. But he admitted to cutting off body parts as part of what prosecutors called a gruesome practice of keeping battlefield trophies.

Staff Sgt. Calvin Gibbs’ surprise testimony came a week after his court martial began. It is the first time Gibbs has publicly presented his version of what happened with the platoon he led from the Army’s 5th Stryker Brigade.

He is the highest-ranking soldier charged in what prosecutors say was a rogue “kill squad” that allegedly targeted Afghan civilians and made the deaths look like casualties of legitimate engagement with insurgents. Gibbs has pleaded not guilty.

Twelve soldiers have been charged in the case. Three have pleaded guilty to murders in the case and agreed to testify against fellow soldiers.

Gibbs has also been charged with removing body parts from his alleged victims, such as teeth and fingers, to keep as souvenirs; planting “drop weapons” to fake attacks on soldiers; and intimidating several of his own unit members to prevent them from speaking out against the unit’s alleged murder plots and rampant drug use.

Gibbs said Friday he never murdered civilians but admitted that in January 2010, he kept fingers he removed from the body of a dead Afghan man against Army regulations. The man is one of three Gibbs is accused of murdering.

Gibbs testified that he later gave the fingers to the soldier that prosecutors say shot the man. “I was numb to the situation,” Gibbs told the court about why he had ripped the fingers from the corpse. “I wasn’t thinking, it’s sickening. I am embarrassed.”

Defense attorney Phillip Stackhouse showed a photo of Gibbs and two other soldiers posing with the body of a different Afghan man. Gibbs has been charged with shooting the man in February 2010 in cold blood and planting an AK-47 near the body to make it look as if the man was an insurgent.

Another soldier — Private Jeremy Morlock — testified previously that Gibbs fired the weapon to make it look like they had come under attack first.

On Friday, Gibbs maintained the man had shot on the three soldiers as they patrolled a village in Kandahar Province and he had fired to protect himself and fellow soldiers. In a calm voice, the sergeant denied testimony from fellow soldiers that he goaded his men into targeting civilians.

But Gibbs admitted to violating Army regulations by cutting fingers off the bodies of dead Afghans and posing for photographs with the corpses.

“People want to prove they were there,” Gibbs replied when asked by Stackhouse why they took the photos. Gibbs testified that he then cut off the man’s finger. “That was the finger he tried to kill me with, I was pretty pissed off about it,” Gibbs told the court.

The sergeant faces a life sentence in military prison if convicted.