The military trial of US army officer Nidal Hasan, charged with killing 13 people at an army post in Texas state, has begun.

Hasan made his opening statement on Tuesday and is representing himself in court. He has admitted that he was the shooter in the attack carried out on November 2009 rampage at Fort Hood, one of the worst mass shootings in US history.

"Witnesses will testify that war is an ugly thing. Death, destruction and devastation are felt from both sides, from friend and foe. Evidence from this trial will only show one side. I was on the wrong side but I switched sides," said Hasan in his two-minute statement.

Hasan, who is confined to a wheelchair after being shot by military police during the attack, is charged with 13 counts of premeditated murder and 32 counts of premeditated attempted murder.

Thirteen officers from around the country who hold Hasan's rank or higher will serve on the jury for a trial that will likely last one month and probably longer.

The former US army psychiatrist has twice dismissed his lawyers, representing himself at the trial, and plans to call on two witnesses. 

He has suggested he wanted to argue the killings were in "defence of others'' - namely, members of the Taliban fighting Americans in Afghanistan.

The trial judge, Colonel Tara Osborn, has so far denied that strategy.

Al Jazeera's Rob Reynolds, reporting from Fort Hood, said the military was not calling the killings a terrorism incident, but rather a "workplace shooting incident".

"In his opening statement Major Nidal Hasan referred to himself as a Mujahideen, and he was fighting to create a perfect religion on earth. He admitted to being the shooter but he said, in effect, that he is a soldier in a war," said Reynolds.

Death penalty

Witnesses say the attack occurred in a building where hundreds of unarmed soldiers, some about to deploy to Afghanistan, were waiting for vaccines and routine checkups.

Hasan walked inside with two handguns, climbed onto a desk and shouted "Allahu Akbar!'' - an Arabic phrase meaning "God is great!'' Then he fired, pausing only to reload.

More than 30 people were wounded in the shooting.

Military law prohibits him from entering a guilty plea because authorities are seeking the death penalty.

Although the Hasan case is unusually complex, experts say the military justice system is unaccustomed to dealing with death penalty cases and has struggled to avoid overturned sentences.

No active-duty US soldier has been executed since 1961.

If Hasan is convicted and sentenced to death there are likely years, if not decades, of appeals ahead.

The Fort Hood shooting trial has been delayed over and over, often due to requests from Hasan.

Source: Al Jazeera and agencies