US in mourning after army base killings

US President Barack Obama led his nation in mourning on Friday as shocked Americans struggled to understand why a Muslim army doctor unleashed a massacre at a US military base, killing 13.

Alleged shooter Major Nidal Malik Hasan, 39, a psychiatrist and specialist in combat stress who had been about to deploy to Afghanistan against his wishes, also wounded 30 people in Thursday's deadly rampage.

Speculation swirled at Fort Hood, Texas on Friday as to whether the alleged shooter Hasan had snapped under the pressure of his job counselling thousands of war-weary troops, or was motivated by deeper convictions.

Obama cautioned against making hasty assumptions as an investigation was launched.

"We don't know all of the answers yet. I would caution against jumping to conclusions until we have all of the facts," he said.

"What we do know is that their families, friends, and an entire nation is grieving right now for the valued men and women that came under attack," Obama said.

He ordered flags to fly at half-staff at the White House and federal buildings, as troops in the US and around the world held a minute's silence to mourn the dead.

Obama would also attend a memorial service due to be held in the coming days, the White House said.

The bodies of those killed were taken to the same mortuary at Dover Air Base in Delaware that handles fallen soldiers from the wars in Afghanistan and Iraq, the Pentagon said on Friday.

Hasan was moved meanwhile from a civilian to a military hospital, in part for security reasons, Fort Hood deputy commander Colonel John Rossi told reporters.

Army chief of staff General George Casey said the attack was "a kick in the gut, not only for the Fort Hood community but for the entire army."

Hasan was shot and seriously wounded by a female civilian police officer who was being hailed as a heroine for ending his deadly rampage. He remained on a ventilator in a civilian hospital Friday.

Witnesses reportedly heard him shout "Allahu Akbar!" (God is great) as he opened fire in a troop processing centre with a semiautomatic weapon and a handgun.

Rossi said investigators believe Hasan fired more than 100 rounds during the incident, and that that accounted for the high number of casualties.

Although "Allahu Akbar" is a Muslim prayer, it has come to be associated with Islamic militants as they carry out attacks or suicide bombings.

A surveillance video aired by CNN showed the major buying breakfast wearing traditional Muslim garb at a base store just hours before the shooting.

The bloodshed dealt a new blow to US forces already under severe strain from repeated combat tours and plagued by a rise in suicides and depression.

Fort Hood, by area the world's largest US military base, has borne the brunt of the wars in Iraq and Afghanistan. Troops based there have suffered the highest number of casualties and have undertaken multiple tours of duty.

Amid the sorrow, the shooting raised delicate questions about Muslim soldiers serving in the US Army, as some Muslim groups feared a huge backlash.

Casey, the army chief of staff, said after a visit to the base that he, too, feared that possibility.

"One of the reasons I told our leaders to keep their people informed and not rush to judgment or speculate until the investigation comes out, I do worry slightly about a potential backlash and we have to be all concerned about that," he said.

Hasan was born in the United States to Palestinian parents who had moved from a small town near Jerusalem.

His cousin Nader Hasan, writing on behalf of the family as Hasan's parents are dead, said they were stunned by Thursday's events and stressed they all considered themselves Americans.

"Our family loves America. We are proud of our country, and saddened by today's tragedy," Nader Hasan said in the message posted on The Washington Post website.

Nidal Hasan's aunt, Noel Hasan, told the daily her nephew had been subjected to harassment about his faith since the September 11, 2001 attacks and had repeatedly sought to be discharged.

An unsigned video praising the attack appeared on a Islamic militant website called the Fallujah forum on Friday, and was swiftly picked up by other sites.