A US soldier convicted of killing five of his colleagues in Iraq in May 2009 was sentenced to life behind bars Thursday and dishonorably discharged.

Army Sergeant John Russell was convicted earlier this week over the murders at a clinic for soldiers suffering from war-related stress at Camp Liberty, the largest US base in Iraq.

Russell, who previously denied responsibility, admitted the killings last month in a plea deal to escape a death sentence, worked out by his lawyers at Joint Base Lewis-McChord (JBLM), in the northwestern US state of Washington.

On Thursday he was jailed for life, reduced to the rank of private and given a dishonorable discharge from the military, military spokeswoman Barbara Junius told AFP.

At the time of the Camp Liberty killings, the incident represented the single deadliest toll on US forces in a month in Iraq, and came at a sensitive moment in the US military's occupation of the country it invaded in 2003.

Russell was on his third tour of duty in Iraq, and his unit was preparing to leave the country.

Due to concerns over Russell's mental state, his commanding officer had ordered about a week before the shooting that his weapon be confiscated and that he get counseling.

After pleading guilty last month, Russell gave an account of the killings for the first time. The victims were three soldiers receiving care at the clinic and two medical officers.

"I just did it out of rage, sir," he told the military judge, Colonel David Conn, describing how he walked from room to room firing at mental health workers and patients.

"I was upset. I do not remember being angry, but I know that everyone who witnessed me outside the combat stress clinic said I looked angry," the Los Angeles Times quoted him as saying.