The American army in Iraq suffered a fresh blow to its image today as it emerged that five soldiers were being investigated for allegedly raping a woman and then murdering her and three members of her family.

The soldiers are accused of burning their alleged victim's body, according to sources quoted by the AP news agency.

Major General James Thurman, commander of coalition troops in Baghdad, has ordered a criminal investigation into the alleged killing of the family of four in Mahmoudiya, south of Baghdad, in March.

"The entire investigation will encompass everything that could have happened that evening. We’re not releasing any specifics of an ongoing investigation," said military spokesman Major Todd Breasseale.

"There is no indication what led soldiers to this home. The investigation just cracked open. We’re just beginning to dig into the details."

A US official close to the investigation said that at least one of the soldiers, all assigned to the 502nd Infantry Regiment, has admitted his role in the attack and been arrested.

It is believed that one of the soldiers has already been discharged and returned to the United States. The others have had their weapons taken away and are confined to Forward Operating Base Mahmoudiyah south of Baghdad.

Two soldiers from the same regiment were killed this month when they were kidnapped at a checkpoint near Youssifiyah.

The official said the killings appear to be unrelated to the kidnappings. He said those involved were all below the rank of sergeant.

He said senior officers were aware of the family’s death but believed it was due to sectarian violence, common in the religiously mixed town.

The killings appeared to have been a "crime of opportunity," the official said. The soldiers had not been attacked by insurgents but had noticed the woman on previous patrols.

Meanwhile, the US Army has reported the deaths of four personnel in the past 24 hours.

A Marine was killed in fighting in the Anbar provice, west of Baghdad today, and three American soldiers died in combat yesterday. One was shot in Mosul, a second killed by a roadside bomb near Balad, 50 miles north of Baghdad, and the third killed by a bomb on foot patrol south of the capital.

The deaths raised to at least 2,533 members of the US military who have died since the Iraq war started in March 2003.

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