The alleged ringleader of a rogue US army unit charged with killing Afghan civilians for fun faces a court martial Friday, 18 months after the slayings.

Staff Sergeant Calvin Gibbs's week-long military trial comes after guilty verdicts were handed down as part of plea deals with three other members of the so-called "kill team."

The previous court hearings exposed grisly evidence about soldiers taking fingers from the bodies as trophies and posing with corpses, evoking memories of abuse at the notorious Abu Ghraib prison on Iraq.

Gibbs faces three charges of premeditated murder as well as a string of accusations of assault, making threats, "conduct prejudicial to good order and discipline," and impeding an investigation.

"If convicted of all charges (and specifications), Staff Sgt. Gibbs faces a maximum punishment of imprisonment for life without the possibility of parole," said a statement from Joint Base Lewis-McChord, south of Seattle.

The court martial is scheduled to last until next Friday, November 4, although officials say its length could depend on how Gibbs's defense lawyers want to proceed.

In all, five soldiers have been accused over the deaths of three Afghans in southern Kandahar province from January to May 2010. The soldiers were from the 5th Stryker Brigade, based in the northwestern US state of Washington.

Three have already pleaded guilty in deals with military prosecutors thought to include agreements to testify against their fellow defendants, including alleged ringleader Gibbs.

In March, Specialist Jeremy Morlock, 23, admitted to involvement in the killings and to using stolen arms to pretend the victims were enemy fighters.

He was jailed for 24 years and dishonorably discharged, but avoided life in prison.

In August Specialist Adam Winfield, 23, was sentenced to three years in prison after pleading guilty to a reduced charge of involuntary manslaughter and illegal use of marijuana.

Then, last month, Private First Class Andrew Holmes, 21, was jailed for seven years in a plea bargain to avoid a 15-year term.

The German news magazine Der Spiegel published three grisly pictures in March, out of a reported cache of 4,000 documents, showing Morlock and Holmes holding up the bloodied head of a corpse.

At a pre-trial hearing in November 2010 for Gibbs, defense attorney Phillip Stackhouse argued that his client had played no role in the killings.

Stackhouse asked the judge presiding over that hearing to weigh Gibbs's assertion of innocence with the testimony from "these dope-smoking soldiers in a combat zone. Who are you going to believe, where does the credibility lay?"

Apart from Gibbs, another soldier, Michael Wagnon, is still awaiting military trial for his alleged role in the killings.

The Pentagon has admitted the allegations and photographs are embarrassing for the US military, but has stressed that such behavior is an "aberration" for a force of nearly 100,000 deployed in Afghanistan.