The latest claim stems from a video broadcast by the BBC showing footage of a number of dead adults and children whom Iraqi police allege were among 11 civilians rounded up and deliberately shot dead by US troops in March.
The BBC quoted a spokesman for US forces in Iraq as saying that an inquiry was underway into the events in Ishaqi, about 100 kilometres north of Baghdad, on March 15 this year.
The BBC received the video from a Sunni Muslim group opposed to US forces. It said the evidence appeared to contradict the US version of events.
US officials said at the time that four people died when US troops became involved in a battle after a tip-off that an al-Qaeda supporter was visiting a house in Ishaqi.
US officials, it added, reported that the home collapsed under heavy fire, killing one suspected militant, two women and a child.
However, a report filed by Iraqi police accused US troops of rounding up and deliberately shooting 11 people in the house, including five children and four women, before blowing up the building.
The BBC video showed several bodies, including those of three children, one of them covered in blood. The BBC's John Simpson said the images clearly show the dead adults and children suffered gunshot wounds.
BBC correspondent Ian Pannell in Baghdad said it had been cross-checked with other images taken at the time of events and is believed to be genuine.
Haditha probe ongoing
The US military is already reeling from allegations of a massacre of 24 Iraqi civilians by US Marines in Haditha in November last year.
US President George W Bush has vowed to punish those responsible if the military inquiry finds US Marines acted improperly.
"One of the things that happens in a transparent society like ours is that there will be a full and complete investigation. The world will see the full and complete investigation," Mr Bush said in trying to defuse a situation that could rival the Abu Ghraib prison scandal of last year.
The Haditha shooting came to light in a Time magazine report in late March which cited an Iraqi human rights group and Haditha residents.
According to the reports, after the bomb detonated, killing one marine, marines barged into a home in the Iraqi village, throwing grenades and shooting several people, including women and children, in cold blood.
Iraqi Prime Minister Nuri al-Maliki has condemned as "a terrible crime" the alleged massacre at Haditha.
Bodies to be exhumed
US investigators probing the Haditha deaths are hoping to exhume the bodies of several victims in search of evidence, a report said.
Naval Criminal Investigative Service (NCIS) experts want to know the distance and the angles at which the shots were fired and the calibre of the bullets, which could help determine how events unfolded, defence officials familiar with the probe told the Washington Post.
The forensic evidence was disregarded at first because the deaths on November 19 last year were reported as caused by a roadside bomb and not treated as crimes.
The officials said the incident was not reported to the NCIS until March 12, nearly four months after it happened, making the work of investigators very difficult because routine forensic evidence of the case is lacking.
Sources told the Washington Post that NCIS investigators have interviewed families of the dead several times and visited the homes where the shootings allegedly took place.
The NCIS probe, the largest homicide investigation since the Iraq war began, is still ongoing "and will remain open until after the findings are adjudicated," NCIS spokesman Ed Buice told the newspaper.
Defence officials say the probe could lead to charges of murder, dereliction of duty and obstruction of justice.
The investigation by the NCIS -- an independent criminal investigative agency that is not in the military chain of command -- is one of two military probes into the incident, which the Iraqi government also plans to look into.
The other investigation by US Army Major General Eldon Bargewell is expected to be delivered to top military commanders by the end of the week, said The Washington Post.
Major General Bargewell's three-month probe will find that false information was given by some US military officers about the case and recommend changes in how US troops are trained, it said.