Hasan, a US-born Muslim, is accused of killing 13 people and wounding 32 others at a Fort Hood readiness facility where many of those shot were preparing to deploy to Iraq or Afghanistan.

Fort Hood was a key deployment point for the wars in those nations, and Hasan had been preparing to leave for Afghanistan with a unit meant to help soldiers deal with mental issues. He was shot by civilian base police during the attack and left paralysed from the chest down.

"I object," he said when the judge ruled he could not use his chosen defence argument, that he was justified in opening fire on soldiers because he was protecting the Taliban in Afghanistan.

After the hearing, Geoffrey Corn, an expert on military law at the South Texas College of Law in Houston, said Hasan's claim was a non-starter.

"I can't see any conceivable set of facts that would raise this defence," Corn said.

Hasan asked Osborn to delay the court martial by three months to let him prepare a new defence strategy and add to the witness lists. Osborn could rule on that request at a hearing scheduled for Tuesday.

Earlier this month, Hasan was granted the right to represent himself at trial. Three military lawyers have been appointed to serve as standby counsel while he leads his own defence.

Opening statements are scheduled for July 1. He could face the death penalty if found guilty.