A US military court has convicted an army squad commander of leading a "kill team" in Afghanistan that murdered unarmed civilians and collected body parts as war trophies.

But Staff Sergeant Calvin Gibbs, 26, could be freed in less than 10 years after receiving a life sentence with the possibility of early parole for murder, assault and conspiracy over the killings of three Afghans in separate incidents staged to look as if the victims were combatants.

In one of the most serious accusations of war crimes to emerge from the Afghan conflict, Gibbs recruited other soldiers to murder civilians he called "savages" after he took over command of a US army squad in Afghanistan's Kandahar province in November 2009. Prosecutors described Gibbs as hunting innocent Afghans "for sport", a view reinforced by the staff sergeant's statement likening the amputation of body parts as trophies to collecting antlers from a deer.

The military prosecutor, Major Rob Stelle, told the court: "Sergeant Gibbs had a charisma, he had a 'follow me' personality. But it was all a bunch of crap, he had his own mission: murder and depravity. No one died before Sergeant Gibbs showed up."

Gibbs was convicted of murder for inciting two soldiers to kill 15-year-old Gul Mudin as he worked in a field. The platoon commander gave a grenade to one of the soldiers, Jeremy Morlock, who threw it at Mudin. A second soldier, Andrew Holmes, then shot the boy. Gibbs played with the corpse of the teenager "as if it was a puppet", Morlock told the trial.

The staff sergeant was also convicted of shooting dead Marach Agha, a man sleeping by a roadside, and then planting a Kalashnikov next to the corpse to make it look as if he was a fighter. He kept part of the victim's skull as a trophy.

Gibbs was convicted on a third count of murder for killing a Muslim cleric, Mullah Adahdad, with a grenade and then shooting him. Two other soldiers, Morlock and Adam Winfield, have already pleaded guilty over their roles in the killing.

Gibbs and other soldiers collected fingers, teeth and other body parts as trophies. They also took photographs of themselves posing next to their dead victims. In one of the pictures Morlock is seen lifting Mudin's head by its hair for the camera and smiling. The soldiers also took ghoulish pictures of themselves with dead combatants.

The jury of five soldiers was shown pages of Facebook messages sent by Winfield to his parents in which he described how Gibbs led the killings. In one exchange with his father Winfield recounted Mudin's killing.

"An innocent dude. They planned and went through with it. I knew about it. Didn't believe they were going to do it. Then it happened. Pretty much the whole platoon knows about it. It's OK with all of them pretty much. Except me. I want to do something about it. The only problem is I don't feel safe here telling anyone. The guy who did it is the golden boy in the company who can never do anything wrong and it's my word against theirs," Winfield wrote.

Winfield later told investigators: "[Gibbs] likes to kill things. He is pretty much evil incarnate. I mean, I have never met a man who can go from one minute joking around, then mindless killings."

The court martial was told that Gibbs had six skull tattoos on his leg to mark up each of his "kills" from tours in Iraq and Afghanistan.

In his testimony Gibbs denied responsibility for the killings, saying the victims all died in legitimate combat. But he did admit slicing off body parts from Afghans, including the fingers of a man, and keeping them or giving them to other soldiers as trophies.

"In my mind I was there to take the antlers off the deer. You have to come to terms with what you're doing. Shooting people is not an easy thing to do," said Gibbs.

The prosecution witnesses against Gibbs included members of his army unit who were also involved in the atrocities. Morlock and Holmes have pleaded guilty to murder and received prison sentences of 24 years and seven years respectively. Winfield pleaded guilty to involuntary manslaughter for failing to prevent other soldiers from attacking Afghan civilians. He was jailed for three years.

Another soldier, Michael Wagnon, is awaiting trial over the killings and collecting human body parts.

The killings came to light in May after the army began investigating an assault on a soldier, Justin Stoner, after he reported to superiors that members of his unit were smoking hashish. Gibbs, Morlock and other members of the platoon are alleged to have beaten Stoner and told him to keep his mouth shut. Stoner reported the beating and told investigators what he knew of the "kill team".

Prosecutors called Gibbs "monstrous" and "savage" and told the military jury he should never be released from prison. But the jurors acceded to the convicted soldier's plea to have the hope of being reunited with his son and sentenced him to life with the possibility of parole after less than 10 years.