The roadside bombing, in Haditha, west of Baghdad, brought the number of American troops killed in the country to 45 in the past two weeks. About 2,700 Iraqis have died in the past three months.
Last night there were unconfirmed reports that US forces were engaged in gun battles after the blast. The Army of Ansar al-Sunna, an Islamic militant group, claimed that they had taken an injured marine hostage after fighting and that his photograph would be published on the internet. The Pentagon, however, denied that any military personnel were missing.
The upsurge in violence comes in the run up to the announcement of the new constitution, which is already mired in controversy over its emphasis on Islamic law, including abrogating some women's rights, and which has been criticised by secular and Kurdish groups.
The deaths are also a blow to plans by President George Bush to scale back American troop levels in Iraq by handing over more security duties to Iraqi forces.
The marines, members of Regimental Combat Team 2, died when their armoured vehicle hit a hidden explosive device.Fighting has intensified in recent weeks in the area along the Euphrates river as US forces step up efforts to seal off the approaches to the Syrian border, which is believed to be a major insurgent crossing point.
The marines launched a series of operations in the region in May and June in hopes of pacifying the area so that Iraqi military and civilian forces could assume effective control of it.
Seven others from the same unit were killed three days ago. The Ansar al-Sunna claimed responsibility for the killings, with masked gunmen displaying helmets, flak jackets and other equipment taken from the dead marines. Yesterday it said in a statement: "The Lions of unity take an American marine hostage and kill eight of the Crusaders in a successful ambush north of Haditha."
In other developments, an American freelance journalist, Stephen Vincent, was killed near Basra in British-controlled southern Iraq and his Iraqi translator, Nouriya Ita'is, shot and left for dead after they were kidnapped by five armed men in a police car as they left a currency exchange.
Mr Vincent, who was writing a book on Basra, had recently written an article for The New York Times saying that the Basra police force had been infiltrated by Shia militia, including members of Muqtada Sadr's Army of Mehdi, and that they had carried out assassinations of former members of the Baath Party.
Mr Vincent's body, with an "execution-style" shot to the head and three gunshot wounds to his chest, was found dumped on a street. Photographs from the morgue showed a red cloth around his neck and plastic handcuffs, of the type used by the police, on his wrists. "Steve, Hay al-Rebaat," an Arabic tag said, identifying Vincent and the area of Basra where his body was found.
Ms Ita'is had been imprisoned by the Iraqi secret police during Saddam's regime and had subsequently worked as a journalist. Her sister said: "She is in intensive care and she is very ill. We are all very scared of what has happened and we are thankful that she is alive."
The casualties came as it was alleged that a CIA organised paramilitary squad had been responsible for torturing prisoners.
The US intelligence agency is also accused of involvement in the brutal beating of a former senior Iraqi officer, Major General Hamed Mowhoush, leading to his death.
Two soldiers from 3rd Armoured Cavalry Regiment, based at Fort Carson, in Colorado, have been charged over the killing and the CIA has said that it is carrying out an internal inquiry into the conduct of one of its officials.
* Italy's state-run RAI television has said that one of its producers was killed in Iraq last month. Adnan al Bayaty, who worked as a freelance producer and translator for RAI, was shot and killed by three armed men at his Baghdad home on 23 July, RAI said.