BAGHDAD, Dec 23: Three US marines and a soldier were among nine people killed in Iraq on Thursday as the US military investigated how a suicide bomber managed to penetrate an army base in a strike that killed 22 people.

The marines were killed while conducting security operations in the restive western Al Anbar province, home to Fallujah, where a few hundred residents entered the battered city for the first time since a massive US-led offensive last month.

In Baghdad, a roadside bomb claimed the life of a US soldier and wounded two others, while mortar attacks killed four civilians and one policeman. The fresh US military casualties came amid a major review of base security operations following Tuesday's attack on an army dining hall in the northern city of Mosul.

The military's second-in-command in Iraq, Lt Gen Thomas Metz, ordered a special investigation of how an infiltrator managed to penetrate the heavily-guarded US military base. "What's going to happen is there will now be an ... investigation. Full force protection will be one of many areas looked at," said a spokesman. -AFP