'American Sniper' Chris Kyle said ex-Marine was 'nuts'

Eddie Ray Routh is accused of shooting two men in the back in 2013

"American Sniper" Chris Kyle described his killer as "nuts" in a text to a friend moments before they were both shot dead by a former Marine, a court was told.

Defence lawyers for Eddie Ray Routh claim the 27-year-old was psychotic at the time of the shooting.

But prosecutors say Mr Routh was aware of what he was doing when he gunned the pair down at a Texas gun range in 2013.

The hit film based on Kyle's memoir is nominated for six Oscars.

The former Navy Seal, who has the most recorded kills of any US sniper, was shot and killed along with his friend Chad Littlefield at a rural shooting range south-west of Fort Worth.

Having retired from the military he had been helping other veterans deal with combat-related stress and mental health issues.

A memoir by Chris Kyle (shown here in 2012) was turned into a film directed by Clint Eastwood

During opening statements, defence lawyer Tim Moore argued Mr Routh had been taken to hospital several times previously for mental health issues and had been diagnosed as psychotic.

His level of psychosis was so high the day of the shooting, Kyle and Littlefield became alarmed, Mr Moore said.

He read to jurors texts that he said the two men exchanged while driving with the former Marine to the shooting range.

"This dude is straight up nuts,'' Kyle texted to Littlefield as Mr Routh sat behind them in the vehicle.

"He's [sitting] right behind me, watch my six," Littlefield texted back, using a military reference for watching one's back.

Chris Kyle's widow, Taya Kyle, was the first witness to be called

Mr Moore told jurors that Mr Routh, who suffered from post-traumatic stress disorder, was under extreme mental distress and was convinced the two men would turn on him.

"He thought he had to take their lives or he was in danger," he said.

The court also heard that Mr Routh was under the influence of marijuana and alcohol at the time of the shooting.

Mr Nash, the Erath County District Attorney, argued evidence in the trial will show that Mr Routh was the person who shot the two men multiple times in the back - and that he knew it was wrong at the time.

The prosecutor said Routh served as a weapons technician in Iraq, but did so in a safe zone.

"The evidence will show that mental illnesses, even the ones that this defendant may or may not have, don't deprive people from being good citizens, to know right from wrong," Mr Nash said.

Defence lawyers said Mr Routh had been more mentally affected by the time he spent helping earthquake relief efforts in Haiti with the Marines in 2010.

He is pleading not guilty to the charge of murder on grounds of insanity. The trial continues.