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 poss i b 1 e Post-Trau m at ic Stress Disorder (PTSD).

All of the dead and wounded were military personnel. At least four of those injured were hospitalised 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.

The US army base remained on lockdown for a couple of hours after staff were ordered to shelter in place, but personnel and family members were later given the all-clear.