The military said one other marine was wounded in the attack which occurred during combat operations near Haditha, 260 km northwest of Baghdad.

The latest deaths bring the number of US military personnel killed in Iraq since the March 2003 invasion to 1,811, according to AFP tally based on Pentagon figures.

Since Monday a total of 21 marines have been killed in western Iraq's Al-Anbar province.

Of these, 20 died near Haditha, while one was killed near the town of Hit, 170 km west of Baghdad.

Ansar al-Sunna, an extremist group linked with the al-Qaeda network, said in an Internet statement on Wednesday that it had killed eight US marines and captured a ninth in western Iraq.

Ansar al-Sunna said in the Internet posting that it had killed some of the marines by "slitting their throats," while others were shot. The statement could not be verified.

It also said that its fighters had captured a ninth US marine "who was wounded in an ambush... near Haditha."

The group vowed to publish more details on the killings and pictures of the "American prisoner" later.

Ansar al-Sunna has claimed a string of attacks in Iraq, including murders of foreign hostages and Iraqis accused of "collaborating" with US-led forces.

Meanwhile an American journalist has been found shot dead in Basra four days after he wrote an opinion piece in the New York Times criticizing the spread of Shia Islamist fundamentalism in the southern Iraqi city.

Witnesses said Steven Vincent and a translator were kidnapped by gunmen shortly after leaving a hotel on Tuesday evening. His body was found later that night, a US diplomat said. A nurse said he had been shot repeatedly in the chest.

Vincent's death appeared to mark the first targeted killing of a Western journalist in Iraq since the US-led invasion toppled Saddam Hussein in 2003.

Other reporters have been killed after being swept up in the violence plaguing the country, but were apparently killed for being Westerners rather than because they were journalists.

"An investigation has been launched to determine who was behind this," said the US diplomat.

A nurse in a Basra hospital said Vincent, a freelance investigative journalist and art critic from New York City who had been working in Basra for several weeks, had been shot three times in the chest.

His Iraqi translator, Nouriya Ita'is, was shot four times but survived. The nurse said she was in a serious condition.

The New York Times opinion piece criticised the failure of British forces to clamp down on what Vincent described as a city that was "increasingly coming under the control of Shi'ite religious groups, from the relatively mainstream ... to the bellicose followers of the rebel cleric Moqtada al-Sadr."

The article also focused on the Basra police force, quoting a police lieutenant as saying a few officers were perpetrating many of what he said were hundreds of assassinations of mostly former members of Saddam's Baath party each month.

Iraqi Arab Sunni leaders have accused the Iraqi government of sanctioning Shia hit squads that work alongside security and police forces. The religious Shia-led government denies the accusations.

Iraq has faced rising sectarian violence since January elections empowered Shias for the first time and sidelined Arab Sunnis, who were dominant under Saddam and are now leading the insurgency.

Sunni Muslim militants from across the Arab world have mounted suicide bombings that have killed hundreds of Shias. Al-Qaeda and other hardline groups have kidnapped more than 150 foreigners. Many were freed and some were shot or beheaded.

Vincent was the author of a book on postwar Iraq and was researching another one about the history of Basra, where around 8,000 British troops are based.