A US Army psychiatrist accused of killing 13 people in an attack on his Texas base will probably plead not guilty and may use an insanity defence at his military trial, his lawyer said.

John Galligan, the civilian lawyer for Major Nidal Hasan, said he was considering an insanity defence among other options.

"Based on the evidence thus far, his mental status must be raised," Mr Galligan said at his office near the Fort Hood base, about 130 miles south west of Dallas.

"Anybody who allegedly engages in conduct that is completely contradictory to his lifestyle and military career - an insanity defence has to be considered."

Hasan is charged with 13 counts of premeditated murder in the November 5 shooting at Fort Hood, and military officials have said they may file more charges.

More than two dozen others were wounded in the shooting spree, which happened at a building where soldiers finalise their wills and are medically screened before they are deployed.

Mr Galligan said military law required his client to plead not guilty if prosecutors sought the death penalty, but said that decision had not been made.

Hasan remains in intensive care at a San Antonio military hospital, where he was taken after being shot during the attack.

At a hearing in his hospital room on Saturday, Hasan was ordered to remain in custody until trial.