"What we have right now is three confirmed suicide vehicle-borne improvised explosive devices," First Lieutenant Kate VandenBossche, a spokeswoman for the US marines, told AFP.

The US marines said three of their number were wounded in the attack, which occurred outside Camp Gannon, a base in Al-Qaim, about 300 kilometresmiles) west of Baghdad in Al-Anbar province.

"The blast was incredible," said police Lieutenant Yasir al-Hadithi adding that US helicopters were flying over the scene of the attack which was followed by the sound of heavy gunfire.

Three civilians, including an eight-year-old boy and an old man, were wounded in ensuing clashes but it was not clear the circumstances or if they involved US troops, a hospital source said.

Qaim, where insurgents are suspected of regularly crossing into Iraq from Syria, is the site of frequent clashes between rebels and US forces.

Despite a massive US-led assault on the former rebel bastion of Fallujah in November, the Iraqi government has been unable to reassert control over Anbar, a stronghold of Islamic fighters and Saddam Hussein loyalists.

In March, the bodies of at least 30 people, most of them believed to be Iraqi soldiers, were found in Qaim. They had been shot.

Local police in areas like Qaim and throughout Al-Anbar province are no longer considered a credible force by the US military due to intimidation and infiltration by the insurgency. The interior ministry is in the process of firing half the entire 12,000-strong force and retraining the rest.

The US military relies on Iraqi troops, mainly Shiite, brought in from outside to help restore law and order to the province.

But areas west of Ramadi are still awaiting a major overhaul of its police and plans are afoot to bring in an Iraqi army division to help subdue the area by the end of year.