A US soldier has shot dead five of his colleagues at a base in the Iraqi capital Baghdad, the Pentagon says.
Two other people were hurt in the shootings and the gunman is in custody, Pentagon officials have said.
An earlier military statement said the incident had happened at Camp Liberty near Baghdad's international airport at about 1400 (1100 GMT).
The White House said US President Barack Obama was shocked by news of the "terrible tragedy".
The president planned to discuss it with Defense Secretary Robert Gates, said spokesman Robert Gibbs.
Any time we lose one of our own, it affects us all
The shooting reportedly occurred at a clinic where troops receive help for personal issues or combat stress.
It is not the first time a US soldier has opened fire on comrades in recent years.
One soldier was sentenced to death in 2005 after killing two officers and wounding 14 other personnel with grenades and a rifle at a camp in Kuwait.
The BBC's Natalia Antelava, in Baghdad, says troops at Camp Liberty had been enjoying a much more relaxed atmosphere in recent months.
She says there have been few attacks on the base recently, so the timing of the shooting will make it particularly shocking to the soldiers there.
It is the deadliest single incident involving US forces since 10 April, when five soldiers were killed by a truck bomb in the northern city of Mosul.
"Any time we lose one of our own, it affects us all," said military spokesman Colonel John Robinson.
"Our hearts go out to the families and friends of all the service members involved in this terrible tragedy."
Earlier this month, a man in an Iraqi army uniform shot dead two US soldiers and injured three others at a base near Mosul.
Iraqi military reports said he was a soldier also working as an imam at a mosque on the base.
US forces are due to withdraw combat troops from Iraq by August 2010.