A soldier with "mental health issues" killed three people and wounded 16 before turning the gun on himself Wednesday at Fort Hood, the US military base devastated by a deadly 2009 shooting rampage.

Base commander Lieutenant General Mark Milley said the gunman, who served in Iraq in 2011, was being treated for depression and anxiety, and being checked for possible Post-Traumatic Stress Disorder (PTSD).

All of the dead and wounded were military personnel. At least four of those injured were hospitalized in critical condition.

"At this time, there is no indication that this incident is related to terrorism, although we are not ruling anything out," Milley told reporters at the sprawling US army installation in Texas.

"We do not know a motive. We do know that this soldier had ... mental health issues, and was being treated for that."

The gunman, who was not immediately identified pending notification of next of kin, used a .45-caliber semi-automatic pistol, which he bought recently and apparently smuggled onto the base.

The assailant shot himself in the head when he was engaged in a parking lot by an armed female military police officer, said Milley.

"A military police officer responded and he was approaching her at about 20 feet," he said.

At that point the gunman "put his hands up, then reached under his jacket, pulled out the (gun). She pulled out her weapon, she engaged, (then) he put the weapon to his head.

AFP