A US army psychiatrist who faces the death penalty on charges of killing 13 people in a 2009 shooting spree on a Texas army base can represent himself at trial, a judge ruled Monday.

Major Nidal Hasan, 42, is also accused of wounding 32 other people in the Fort Hood massacre -- people who could now be called as witnesses he would then be allowed to question in court.

Jury selection is set to begin Wednesday for the trial over the shooting, which jolted the US military and prompted calls for stronger safeguards against possible internal security threats and "homegrown" terror attacks.

Military judge Colonel Tara Osborn agreed to let Hasan represent himself at the trial -- scheduled to begin on July 1 -- after a military doctor testified that he was fit to handle the physical strain.

Hasan was paralyzed after being shot by police officers as they tried to halt the carnage. He had previously been ruled mentally fit to stand trial.

Osborn also ruled that Hasan's current defense team will sit with him throughout the trial and act as "stand-by counsel," Fort Hood said in a statement.

"It's highly unusual and extremely rare for an accused to defend himself in a capital murder trial in a military court," said Morris Davis, a law professor at Howard University and retired air force colonel.

Osborn will likely give Hasan a "wide berth" during the trial but "won't let him make a mockery of the process," Davis told AFP.

She will also have to "make sure that it's clear and on the record that he understands what he's doing, and recognizes the consequences and the pitfalls of being his own attorney, so that there won't be an appellate issue afterwards," he added.

Hasan has repeatedly attempted to plead guilty but the request was denied because prosecutors were unwilling to waive the death penalty, and military law does not permit people to plead guilty to a capital offense.

The FBI alleges Hasan had contacts with firebrand US-born cleric Anwar al-Awlaqi, a key leader of Al-Qaeda in the Arabian Peninsula who was killed in a 2011 drone strike.

The Yemeni-based cleric was also believed to have been instrumental in planning the botched plot by a Nigerian student dubbed the "underwear bomber" who tried to blow up a US-bound airliner on December 25, 2009.

Umar Farouk Abdulmutallab also convinced a judge to let him represent himself and used opening statements of his 2011 federal trial in Detroit as an opportunity to espouse his beliefs before abruptly pleading guilty.

He was sentenced to life in prison with no chance of parole.

"It's always a mistake for anyone to represent themselves," said Gerald Zerkin, a public defender who represented Zacarias Moussaoui after a judge ruled he could no longer represent himself on charges of helping to plot the attacks of September 11, 2001.

Hasan, who was born in Virginia to Palestinian parents and raised in the eastern US state, had attended a mosque in 2001 where Awlaqi worked and is believed to have continued to communicate with the radical cleric.

The Pentagon has refused to call the shooting rampage at Fort Hood a terrorist attack, classifying it instead as an act of workplace violence.

Hasan was set to deploy to Afghanistan weeks after the attack.

He filed a motion Monday requesting that the trial be delayed by three months so he can prepare a "defense of others" trial strategy and revise the witness list. Osborn is expected to rule on that request Tuesday.