The ringleader of a rogue US army unit in Afghanistan "betrayed" the American people by killing Afghan civilians for sport, prosecutors said as a court-martial neared its climax.

A five-person military panel could give its verdict as early as Thursday in the case of Staff Sergeant Calvin Gibbs, the alleged leader of a so-called "kill team" three members of which have already pleaded guilty.

In a closing argument Wednesday, Major Robert Stelle dismissed Gibbs's claims that his unit was responding to legitimate attack when they killed the civilians, before removing body parts as trophies and taking photos with the corpses.

"This is a case about betrayal, the ultimate betrayal... Staff Sergeant Gibbs betrayed his folk, he betrayed his unit, and with the flag of his nation emblazoned across his chest, thousand miles from home, he betrayed his nation," he said.

Gibbs, 26, faces life behind bars if convicted on charges including three counts of premeditated murder in southern Afghanistan between January and May last year, in a scandal that has threatened Abu Ghraib-style embarrassment for the US military.

Specifically he is accused of setting up the killings, and then planting weapons on the dead civilians' bodies to make it look like they were fighters, before removing fingers and teeth to show their colleagues.

Gibbs' court-martial started last week, and in an unexpected appearance Friday he claimed his unit was engaged in genuine combat -- while admitting that he took fingers and teeth from the dead corpses as what prosecutors described as "trophies."

"In my mind, it was like keeping the antlers off a deer you'd shoot. ... You have to come to terms with the things you're doing," he said, explaining the removal of body parts.

But the military prosecutor dismissed those claims during a closing argument lasting over an hour at the Joint Base Lewis/McChord, south of Seattle, home to the 5th Stryker Brigade.

"Selling the engagement as legitimate was part of the plan," said Stelle, calling Gibbs's stories "fundamentally implausible."

Gibbs' lawyer countered by attacking the credibility of prosecution witnesses including Private Jeremy Morlock, one of three members of the "kill team" who received lighter sentences by pleading guilty and agreeing to testify against Gibbs.

As well as attacking Morlock's statements, defense attorney Phil Stackhouse used over two hours pointing out discrepancies in the evidence submitted in an attempt to build reasonable doubt in the minds of the five-soldier panel.

One of the largest discrepancies revolved around the barrel length of an automatic weapon allegedly "dropped" on a victim to make the killing appear justified.

The prosecution alleges that the AK-47-type gun was stuffed into Gibbs' backpack, but the defense said it was too big to fully close the pack, casting doubt on whether Gibbs could have used his pack to conceal the weapon before one of the killings.

Morlock testified earlier in the trial that Gibbs placed the weapon near the victim later after an alleged killing later that day.

Stelle called the idea that Morlock and the other kill team members were willing to plead guilty to murder in an effort to foist blame onto Gibbs as "patently ridiculous."

The court martial is set to resume Thursday, possibly for its last day.