A US soldier accused of plotting an attack on a military base after fleeing his post as a Muslim conscientious objector went on trial Tuesday wearing a surgical mask and manacled to the floor.

Courtroom security agents behind him wore protective goggles, an apparent reaction to an incident in which the soldier, Naser Jason Abdo, who claims to be HIV positive, bit his lip and spat blood at law enforcement officers.

Prosecutors called the first of 43 witnesses to the stand in a bid to show that Abdo, who fled his post in Kentucky, was gathering bomb-making materials and weapons to attack soldiers and their families at the Fort Hood base in Texas, the scene of a deadly shooting rampage in 2009.

One witness said Abdo told him that the assault was intended to show support for Army Major Nidal Malik Hasan, a Muslim psychiatrist accused of killing 12 soldiers and a civilian in the 2009 shooting, which also wounded 32 others.

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

FBI agent Charles Owens said that Abdo told him in an interrogation session that "he wanted to do it for the sake of the men and women of Afghanistan, that they had been wronged."

Abdo was arrested July 27 at a discount hotel in the nearby Texas town of Killeen. Police and federal agents have previously testified that they found a handgun and enough gunpowder to make at least one bomb.

They also discovered directions from an Al-Qaeda magazine on how to build an explosive device.

Prosecutors mounted a detailed case, mixing a trail of receipts and time-stamped videos of Abdo with testimony from a number of workers who encountered him.

They said he planned to detonate a bomb in a crowded Chinese restaurant not far from Fort Hood, and then gun down soldiers, their families and civilians as they fled.

"He referred to civilians as collateral damage," Owens said.

Defense attorney Zachary Boyd countered that prosecutors couldn't prove that his client intended to kill anyone, and that no explosive device was ever built.

"I want the jury to focus not just on the evidence, but the law," Boyd said in his opening statement.

Prosecutor Gregg Sofer told jurors that Abdo had intended to kidnap a soldier and execute him on video when he was still in Kentucky.

"He had already acquired a body bag, a stun gun, a cattle prong," Sofer told the jury.

But the plan fell apart and Abdo fled, leaving his Cadillac, body bags, a green body bag carrier and bleach to clean up the crime scene.

A hood, three handcuff boxes, batteries for the prong, his car keys and identification papers were also found.

"My heart was racing," Oak Grove Police Sergeant Victor Lynch told the court. "I was thinking somebody was in danger."

Abdo bought a .40-caliber handgun and two extended round clips from a man in Nashville and paid $315.05 in cash at a Dallas-area department store for items that prosecutors said could be used to make a bomb, including electrical wiring, clocks and a pressure cooker.

He then took a four-hour cab ride from North Texas to Killeen, where he bought smokeless gunpowder in a local gun store.

A federal forensic analyst testified that the powder, typically used in fireworks, burns slowly but could be used to detonate a bomb.

Prosecutors displayed receipts for the goods and showed videos of Abdo at the department store, his Killeen hotel and at the gun store a few miles down the road.

Along the way, he aroused suspicion that twice prompted police investigations and ultimately led to his arrest.

One store employee recalled telling Abdo to have a nice day after he paid $256.44 cash for a number of items that included the smokeless gunpowder.

Bothered after the exchange, those at the store later called police.

"I just had a feeling," the worker said.