Four people, including the shooter, were killed and at least 16 others injured Wednesday in a shooting at the US Army base of Fort Hood in Texas, the commander of the base said.

In the incident, which lasted between 15 and 20 minutes, the suspect, a soldier who served in Iraq and was being treated for mental health problems, drove to two buildings at the base and opened fire before being stopped by military police, Lieutenant General Mark Milley told reporters.

"There is no indication that this incident is related to terrorism," Milley said, adding that the gunman shot himself in the head with a .45-caliber pistol recently purchased outside the base.

"He was currently under diagnosis for PTSD (post-traumatic stress disorder) but had not yet been diagnosed," he said.

The soldier was stationed in Iraq for four months in 2011 and had reported suffering "depression, anxiety and a variety of other" complaints, he added.

Milley declined to identify the shooter until his family was notified, saying all the wounded and killed were military personnel.

The injured people, with three in critical condition, are being treated at local hospitals, he said.

The military base was on lockdown for several hours as the incident unfolded, but an all-clear was given late Wednesday.

U.S. President Barack Obama said he was "heartbroken" that another shooting occurred at the military base and described the situation there as fluid.

"We are going to get to the bottom of exactly what happened," Obama told reporters in Chicago, where he is traveling for Democratic fundraisers. "We're heartbroken that something like this might have happened again."

Fort Hood was the scene of a shooting rampage on Nov. 5, 2009, when Nidal Malik Hasan, an Army psychiatrist, opened fire at a soldier readiness center at the base, killing 13 people and wounding more than 30 others. It is the worst shooting that has ever taken place at an American military base.