WASHINGTON (AFP) - Armed gunmen Thursday killed seven people and wounded at least 12 others in a bloody rampage at Fort Hood military base in Texas, US officials and media said.

Security forces were hunting for a gunman who remained at large, as both the Pentagon and local police confirmed to AFP that there had been a shooting on the largest US military base in the world.

It was not immediately clear whether the victims were civilians or military personnel stationed on the base with up to 40,000 troops most of whom have seen extensive tours of duty in Iraq.

President Barack Obama had been informed of the shooting at the base in Killeen, in central Texas, a White House official said.

Sergeant Major Jamie Poston, a Fort Hood spokesman told CNN television, the military was "not sure right now" what had happened.

"At this point we're looking for the other shooter... Emergency services responded. Have evacuated a number of wounded," he added, saying he could not confirm the number of victims.

"We are on the lookout for the second shooter," Poston said.

One suspect was in custody, Master Sergeant Tim Volkert at Fort Hood told AFP. "We're trying to confirm the exact numbers" of dead and wounded, he added.

But a Killeen police department spokeswoman warned there were still suspects at large. "All the suspects are not in custody at this time," she told AFP. "I know they have active shooters out there."

She would not give any more details, but local and national media reported that seven people had been killed and 12 others wounded.

MSNBC reported speculation that there may be a third gunman. One of the shooters at large was believed to have a high-powered sniper rifle, and was holed up in a building surrounded by SWAT teams, MSNBC said.

It was not immediately clear if it was an attack on the base, or whether the shootings were carried out by US soldiers.

Local congressman John Carter said the gunfire had erupted ahead of a graduation ceremony in the soldier readiness center.

"I had a man on the scene, who is my regional director and former chaplin at Fort Hood, waiting to go to a graduation ceremony when a soldier came running up to him saying, "sir, don't go over there. They are -- somebody is shooting over there," Carter told on MSNBC.

"When the soldier ran by him, he saw the soldier didn't know it, but he was wounded. So, he went into the building and they stopped him, because he had been shot.

"He heard small arms and some rifle fire while he was there and that he thought they had one person that they had caught but there may be more, he didn't know. He understood there was more than one, that was all he said."

Fort Hood has been working to rehabilitate many soldiers suffering from post-traumatic stress syndrome, Carter added.

A message on Fort Hood's website -- the headquarters of the Army 3rd Corps, the 4th Infantry Division and the 1st Cavalry Division -- said it was closed, but gave no further details. All those units have seen extensive duty in Iraq.

"Effective immediately. Fort Hood is closed. Organizations/units are instructed to execute a 100 percent accountability of all personnel. This is not a Drill. It is an Emergency Situation," the website.