Twelve people have died and 31 more are wounded following a shooting at an army base in central Texas.

However, the gunman is not among the dead.

The U.S. Army said on Thursday that the alleged shooter is in hospital and was not fatally shot by authorities, as previously reported.

There was confusion at the hospital, the spokesman said. The death toll remains at 12 because another person has died. 

The alleged gunman has been identified as Maj. Malik Nadal Hasan. He was a military mental health doctor who was soon to be deployed, officials said on Thursday.

An army spokesperson in Washington D.C. says gunfire first erupted around 1:30pm local time Thursday at the personnel and medical processing centre near Killeen, which is just southwest of Waco.

Two additional soldiers were taken into custody, but they have since been released.

The violence at Fort Hood is believed to be the worst mass shooting to ever take place at a U.S. military base.

The base was immediately placed on lockdown with units on hand asked to account for all personnel.

Fort Hood is the army's largest U.S. post, home to roughly 40,000 troops.

President Barack Obama called the shootings "tragic" and "a horrific outburst of violence" at a press conference Thursday evening, expressing his condolences for the shooting victims.