BAGHDAD - An American soldier opened fire at a counselling centre on a military base Monday, killing five fellow soldiers before being taken into custody, the U.S. command and Pentagon officials said.

Although it was unclear what prompted the shooting, the incident draws attention to the issue of combat stress and morale after six years of war as the mission of the 130,000-strong force transforms to one of training and mentoring the Iraqis.

Attacks on fellow soldiers, known as fraggings, were not uncommon during the Vietnam war but are believed to be rare in Iraq and Afghanistan.

President Barack Obama said in a statement that he was "shocked and deeply saddened" by the report, adding that "my heart goes out to the families and friends" of all those involved "in this horrible tragedy."

After a meeting with U.S. Defence Secretary Robert Gates, Obama said he would ensure "that we fully understand what led to this tragedy" and would do everything possible "to ensure that our men and women in uniform are protected as they serve our country so capably and courageously in harm's way."

A brief U.S. statement said the assailant was taken into custody following the 2 p.m. shooting at Camp Liberty, a sprawling U.S. base on the western edge of Baghdad near the city's international airport.

The statement said nobody else was hurt, but a senior defence official in Washington said three people were wounded. The names of the victims and shooter were not released.

At the Pentagon, Gates said that "we're still in the process of gathering information on exactly what happened."

Gates said if as suspected one serviceman turned his gun on others, that would be "a cause of high and great concern" and would get the highest level of attention at the Defence Department.

Pentagon officials said the shooting happened at a stress clinic, where troops can go for help with the stresses of combat or personal issues. It was unclear whether the gunman and his victims were workers at the clinic or were there for counselling. Soldiers routinely carry weapons on Camp Liberty and other bases, but they are supposed to be unloaded.

Adm. Mike Mullen, chairman of the Joint Chiefs of Staff, said "The tragedy occurred in a place where individuals were seeking help."

While Mullen said he did not have all the details on the shooting, he said it underscores the need for troops to spend more time at home between combat deployments.

"It does speak to me about the need for us to redouble our efforts in terms of dealing with the stress," he said.

The U.S. military is coping with a growing number of stress cases among soldiers in Iraq and Afghanistan - many of whom are on their third or fourth combat tours. Some studies suggest that about 15 per cent of soldiers returning from Iraq suffer from some sort of emotional problems.

With violence declining, many soldiers face new challenges trying to shift from fighting a war to supporting the Iraqis - tasks that often require skills in which they have not been trained.

Adding to the stress, there have been several incidents recently when men dressed as Iraqi soldiers have opened fire on American troops, including an attack in the northern city of Mosul on May 2 when two soldiers and the gunman were killed.

Separately, the military announced Monday that a U.S. soldier was killed a day earlier when a roadside bomb exploded near his vehicle in Basra province of southern Baghdad.

The death toll from the shooting at the counselling centre was the highest for U.S. personnel in a single attack since April 10, when a suicide truck driver killed five American soldiers with a blast near a police headquarters in Mosul.

"Any time we lose one of our own, it affects us all," U.S. spokesman Col. John Robinson said. "Our hearts go out to the families and friends of all the service members involved in this terrible tragedy."

There have been several previous fragging incidents in the Iraq war.

-Last September, Sgt. Joseph Bozicevich, 39, of Minneapolis was detained after allegedly killing two members of his unit south of Baghdad. The case remains under investigation.

-In April 2005, Army Sgt. Hasan Akbar was sentenced to death for killing two officers in Kuwait just before the U.S.-led invasion of Iraq in 2003.

-In June 2005, an Army captain and lieutenant were killed when an anti-personnel mine detonated in the window of their room at the U.S. base in Tikrit. National Guard Staff Sgt. Alberto Martinez was acquitted in the blast.

-Spc. Chris Rolan, an Army medic, was sentenced to 33 years in prison in 2007 for killing a fellow soldier after a night of heavy drinking in Iraq.

-In 2008, Army Cpl. Timothy Ayers was sentenced to two years and four months in prison after pleading guilty to involuntary manslaughter in the fatal 2007 shooting of his platoon sergeant in Iraq.

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Associated Press Writers Pauline Jelinek in Washington and Ross Bynum in Savannah, Ga., contributed to this report.

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