Twelve people have died and 31 have been wounded after at least one US soldier opened fire at the Fort Hood military base in Texas, US authorities say.
One soldier, identified as Major Hassan Malik, was shot dead by police after opening fire with two handguns in a busy processing centre where troops go before being deployed on overseas missions in Iraq and Afghanistan.
Two other US soldiers were arrested in connection with the shootings, however Lieutenant General Robert Cone said it was not yet known if they had fired any shots.
Texas senator Kay Bailey Hutchison said Major Malik was about to be deployed to Iraq, while CNN reported he was a mental health care professional.
"It's a terrible tragedy ... it's stunning," Lieutenant General Cone said.
An Army spokesman at the Pentagon said the shootings began about 1.30pm on Thursday (0630 AEDT, Friday) at a personnel and medical processing centre.
A second incident took place at a theatre on the sprawling base in Killeen.
Earlier media reports said one of the gunmen had a high-powered sniper rifle.
All the dead were killed in the initial shooting incident at the processing centre, Lieutenant General Cone said. Local media had reported that the areas where the shootings took place were frequented by the family members of soldiers.
Lieutenant General Cone said no children were among the dead. But one was a civilian policeman, he said.
The 31 wounded were taken to local hospitals in Texas. A number of ambulances were shown on local TV leaving the base with their sirens blaring.
Texas hospital Scott and White, in Temple, updated its website to say it was receiving victims from the army base who were "suffering from gun shot wounds".
Local congressman John Carter confirmed on MSNBC that the gunfire had erupted during a graduation ceremony in the soldier readiness centre.
Fort Hood is the largest US military base in the world and had been working to rehabilitate many soldiers suffering from post-traumatic stress syndrome, Carter said. Many of the soldiers at Fort Hood had been on repeated deployments to Iraq and Afghanistan, MSNBC said.
The website of the base in central Texas posted an alert that says: "Effective immediately. Fort Hood is closed. Organisations/units are instructed to execute a 100 per cent accountability of all personnel. This is not a drill. It is an emergency situation."
US President Barack Obama described the incident as a "horrific outburst of violence".
"It's tough enough when we lose [soldiers while they are fighting] overseas ... it's horrifying that they should come under fire at an army base on American soil," he said.
He said his thoughts were with the wounded and the "families of the fallen", and paid tribute to the "selfless and courageous" devotion of the soldiers to their country.
Bridget Donovan, a resident on the base, described her terror as the emergency unfolded..
"locked in my post housing. scared. dont know where the shooters are," she wrote on Twitter.
"all i hear are sirens telling us to stay indoors. can't hear any gunfire.
"i'm safe, but scared. i have my husband and many friends on post. worried about them."
Schools in the area were also on lock-down, MSNBC said.
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