A US Army psychiatrist, apparently disgruntled at being deployed in Iraq, went on a "horrific" shooting spree targeting fellow soldiers at America's largest military base near Houston, leaving 12 persons dead and 31 injured, in the worst mass killing ever at a US defence post.

The gunman was identified as 39-year old Major Nidal Malik Hasan, an American national of Jordanian descent, who was about to be deployed to Iraq, officials at the Fort Hood military base in St Antonio in central Texas, said.

Hasan opened fire with two handguns at a military processing centre, where soldiers go for a last-minute medical check up before being sent on deployment.

The soldiers were making their final preparations for deployment to Iraq, Fort Hood's base commander Lt Gen Bob Cone told reporters late last evening.

The attacker was shot several times in exchange of fire and was captured. "His death is not imminent," said Cone.

Military officials said that the soldier attacked other troops at the Soldier Readiness Processing Center and the adjacent Howze Theater on the sprawling Central Texas post.

Ten of those killed were soldiers, Cone said.

Two other soldiers were taken into custody but later released after being interviewed.

Cone did not speculate about a motive, but the Army released a statement saying it did not believe the shootings were an act of political terrorism.

The incident that sent shock waves across the country was termed by Obama as "a horrific outburst of violence".

"It is difficult enough when we lose these brave Americans in battles overseas... It is horrifying that they should come under fire at an Army base on American soil," Obama said in Washington.

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