WASHINGTON, Nov 6 US President Barack Obama urged his nation on Friday not to jump to conclusions on the motive behind the shooting at a Texas army base as American Muslims braced for a possible backlash.

“We don't know all of the answers yet, and I would caution against jumping to conclusions until we have all of the facts,” Mr Obama cautioned.

Major Nidal Malik Hasan, 39, a US-born Muslim and army psychiatrist, opened fire at the Fort Hood base on Thursday, killing 13 and wounding 30 people. He was due to be deployed soon to Afghanistan and was strongly opposed to the deployment.

The US president, however, promised his nation that “we will make sure that we get answers to every single question about this horrible incident”.

Mr Obama also said that he had met FBI Director Robert Mueller and officials from other agencies “to discuss their ongoing investigation into what caused one individual to turn his gun on fellow servicemen and women”.

Meanwhile, a Muslim advocacy group, the Council on American-Islamic Relations, urged Muslims to take necessary precautions against possible hostile reactions.

“Unfortunately, based on past experience, we also urge American Muslims, and those who may be perceived to be Muslim, to take appropriate precautions to protect themselves, their families and their religious institutions from a possible backlash,” said a statement issued by CAIR's Washington office.

“We most vigorously condemn the contemptible attack causing horrific loss of so many precious lives. Such unconscionable attack cannot be justified in the name of any religion, ideology, or cause,” said another group called the American Muslim Alliance.

“American Muslims stand with fellow citizens in offering prayers for the victims and heartfelt condolences to the bereaved families.”

CAIR urged all American “political and religious leaders and media professionals to set a tone of calm and unity”.

President Obama said he had ordered all flags at the White House and other Federal buildings to be flown at half staff until Veterans Day on November 11. The shooting began at about 1330 (1930 GMT) on Thursday at a personnel and medical centre at Fort Hood -- the largest US military base in the world, home to about 40,000 troops.

The commander of the base, Lt-Gen Robert Cone, told NBC News that, according to eyewitnesses, Mr Hasan had shouted “Allahu Akbar!” before opening fire.

He was shot four times during the attack and is currently being treated in hospital under armed guard.

The policewoman who shot him first -- named as Kimberly Munley, 34 -- was among those wounded.

The suspect was transferred to the Texas base in July. Reports suggested that he had been increasingly unhappy in the military.

His cousin told US media that Major Hasan opposed an imminent deployment overseas, describing it as his “worst nightmare”.

He also said that Major Hasan had been battling racial harassment because of his “Middle Eastern ethnicity”.

Experts appearing on various US television channels said the incident could put pressure on Muslim American soldiers.

“The incident will make people distrust them. Other soldiers will be scared of them and their superiors may be reluctant to deploy them at sensitive positions,” said one such expert.

The New York Times said the FBI had been investigating internet postings by a man called Nidal Hasan that appeared to back suicide bombings -- but said it was not clear whether it was the suspect.

The Washington Post reported that Fort Hood leads US Army bases in the number of suicides since the Iraq invasion. This year, 117 active-duty army soldiers reportedly committed suicide, with 81 of those cases confirmed.

Ten suicides have been reported at Fort Hood this year; more than 75 of its personnel have committed suicide since 2003. Fort Hood's high number of suicides is also linked to the fact that it is the army's largest base, the Post writes.

“The psychological toll on the all-volunteer force today is unprecedented ... making the health of the force a major consideration in President Obama's current deliberations over sending more US troops to Afghanistan,” the Post noted.

The newspaper also reported that an estimated 30 per cent of those returning from combat suffer depression, anxiety and post-traumatic stress, which grow worse with repeated tours, the constant exposure to danger and the sights, smells and emotions of seeing others killed or wounded, army mental health surveys have found.

Those who treat the mentally wounded, including doctors such as Major Nidal Malik Hasan, are not immune. Therapists who treat patients for post-traumatic stress often experience symptoms vicariously after hearing of battle horrors, the Post noted.

The San Antonio Express-News, a newspaper published from the area near the army base, reported that the problems at Fort Hood were so serious that its former post commander, Lt-Gen Rick Lynch, launched a series of initiatives designed to combat soldier and family stress.

Soldiers with the 4th Infantry Division told the newspaper in Baghdad last year that they routinely used alcohol to excess and had problems with their marriages. Many were on their third tour of Iraq, and some were on their fourth.