The American soldier accused of killing 16 Afghan civilians, most of them children, had been trained as a sniper and recently suffered a head injury in Iraq, U.S. officials said Monday.

The name of the suspect, a married, 38-year-old father of two, has not been released.

U.S. Secretary of Defence Leon Panetta said the soldier may face capital charges, and that the U.S. must resist pressure from Washington and Kabul to change course in Afghanistan because of anti-American outrage over the shooting.

'We seem to get tested almost every other day with challenges that test our leadership and our commitment to the mission that we're involved in,' Panetta told reporters travelling with him to Krygzystan. 'War is hell.'

A U.S. official said that during a recent tour of duty in Iraq, the suspect suffered a head injury in a traffic accident. The official spoke on condition of anonymity because the matter is under investigation. The vehicle accident was not a combat-related event, the official said.

There was no available indication about the extent of the injury, or whether his injury could be linked to any abnormal behaviour afterward.

Two U.S. officials speaking on condition of anonymity said the suspect had been trained as a sniper.

US soldier Robert Bales kills 16 coverup? in 4 rooms then moved all in 1 room 'put-on fire'.