A US Army psychiatrist admitted Tuesday to opening fire on fellow soldiers at the Fort Hood military base as he took charge of his defense at a high profile trial.

Major Nidal Hasan's opening statement lasted just a couple of minutes, as he explained to a jury of 13 Army officers that the troops he killed were casualties of war after he "switched sides."

"The evidence will clearly show I am the shooter," declared Hasan, referring to the 2009 attack at the Texas garrison, which jolted the American military and shocked the nation.

Paralyzed from the waist down after being shot by police trying to stop the carnage, Hasan was calm and collected Tuesday as he monitored the proceedings from his wheelchair in a green camouflage uniform.

Sporting a thick beard responsible for delaying the trial by months as he fought to win the right to violate the military's grooming rules, Hasan fired his lawyers and is representing himself.

He has previously admitted to killing 13 people and wounding 32 others at Fort Hood, and faces the death penalty if convicted. Now aged 42, Hasan was due to deploy to Afghanistan weeks after the attack. He has said he shot the soldiers to protect his fellow Muslims from an "illegal" war.

His opening statement reiterated his radical views.

"We, the mujahedeen, are imperfect Muslims trying to establish a perfect religion in the land of the supreme God," Hasan said. "I apologize for any mistakes that I made in this endeavor."

Military law prohibits Hasan from pleading guilty to a capital offense and so he has been given the opportunity to try to convince the jury that he does not deserve death for his actions.

The Fort Hood killings prompted calls for stronger safeguards against internal security threats and "homegrown" terror attacks.

Nearly four years after being attacked in what should have been the safety of a protected base, survivors face being cross-examined by Hasan.

Military judge Colonel Tara Osborn has insisted that Hasan does not use the high-profile trial as a platform to espouse extreme views and that he treats witnesses with respect.

But Shawn Manning, a mental health specialist in the same unit as Hasan who was shot six times, said he was dreading the prospect of being cross-examined by the shooter.

"A guy who tried to murder you and your friends, and you have to be cordial and nice, it is going to be difficult," Manning told AFP as he prepared to testify later in the week.

"In a lot of ways, I hope he doesn't ask me any questions, but I've prepared myself."

Manning is the lead plaintiff in a lawsuit urging the military to reclassify the shooting as "terrorism" instead of the current designation of "workplace violence," which offers less compensation to victims.

Osborn has barred prosecutors from mentioning terrorism as a motive and prohibited Hasan from using a "defense of others" strategy to justify his actions.

Hasan attempted to bring up allegations of war crimes committed by US soldiers in Iraq while cross-examining a witness, but Osborn ordered his former supervisor not to respond to questions about e-mails Hasan had sent him prior to the attack.

Three weeks before the shooting, according to prosecutors, Hasan told a doctor: "They have another thing coming if they think they are going to deploy me."

As he prepared to kill as many soldiers as possible, Hasan read jihadist writings by Taliban leaders and wrapped ammunition magazines in paper towels so people wouldn't hear them clinking in his pockets, prosecutors said.

Born in the eastern US state of Virginia to Palestinian parents, Hasan joined the Army in 1995.

It was during a residency at Walter Reed Army Medical Center from 2003 to 2006 that he first exhibited signs of radical Islamic views, according to an FBI report entitled "A Ticking Time Bomb."

Hasan attended a mosque where radical US-born cleric Anwar al-Awlaki -- a key figure in Al-Qaeda in the Arabian Peninsula until his death in a 2011 drone strike in Yemen -- worked in 2001.

He exchanged emails with Awlaki in the months leading up to the shooting in which he questioned the morality of killing soldiers if they intended to attack Muslims. Awlaki later called Hasan a hero.