Recent figures from the campaign group Iraq Body Count put the minimum number of civilians killed in Iraq since the US-led invasion three years ago at between 33,710 and 37,832.
By David Gritten
BBC News website
Foreign troops are immune from prosecution by Iraqi law
Although many of those deaths were caused by insurgent attacks, multinational forces stationed in Iraq have been responsible for a significant number post-invasion.
Hundreds of civilians have been killed during major offensives by US-led forces against insurgents in cities such as Falluja, and many others have died after lethal force was used at military checkpoints.
Military commanders have said those killed were "collateral damage" or the unfortunate victims of "crossfire" between their troops and militants.
But the announcement that US military investigators have flown to Iraq to study allegations that their troops deliberately shot dead at least 15 civilians in Anbar Province in November has cast doubt on some of those claims.
'Riddled with bullets'
A US statement at the time said the civilians, including seven women and three children, died in a roadside bomb explosion that also killed a marine in the western town of Haditha.
But survivors and those who saw the bodies said the account was not true.
The victims' relatives say US troops went on the rampage in Haditha
"Their bodies were riddled with bullets, there was evidence that there had been gunfire inside their homes, there were blood spatters inside their homes," Bobby Ghosh, a journalist who took up the case for Time magazine, told the BBC.
"It was quite clear that these people were killed indoors, which couldn't possibly have happened if they'd been involved in a roadside blast."
An initial military inquiry found the two families had indeed been shot dead in their homes by the marines, but it described the deaths as "collateral damage".
The report has now prompted the US Naval Criminal Investigation Service (NCIS) to determine the motives behind the killing.
The NCIS will have to decide whether the civilians were killed by accident or were targeted by the marines as an act of revenge in a potential war crime.
Several American veterans of the war in Iraq have told the BBC's Newsnight programme that the marines' reaction to the roadside bomb attack in Haditha was not an isolated incident.
Specialist Michael Blake, who served in Balad, said it was common practice to "shoot up the landscape or anything that moved" after an explosion.
Another veteran, Specialist Jody Casey, who was a scout sniper in Baquba, said he had also seen innocent civilians being killed.
Bombs "go off and you just zap any farmer that's close to you", he said.
Mr Casey said he did not take part in any atrocities himself, but was advised to always carry a shovel. He could then plant this on any civilian victims to make it look as though they were digging roadside bombs.
The US and British governments say the fact the allegations are being investigated at all shows that progress has been made in Iraq.
UK International Development Minister Hilary Benn welcomed the inquiry and said it was important that the perpetrators were being brought to justice.
"The big difference between now and the 30 years that people endured under Saddam is that when things happened nobody was called to account, there was no due process," he said.
Although human rights groups have also welcomed the launch of the inquiry, they are quick to point out that the multi-national forces have investigated only a minority of the reports alleging the unlawful or deliberate killing of Iraqi civilians.
Nicole Choueiry, a spokeswoman for Amnesty International, told the BBC News website that those investigations which had taken place had often been inadequate and shrouded in secrecy.
The victims' families are also often unaware of how to apply for compensation.
There are no governmental or judicial bodies in Iraq to investigate human rights violations and the activities of international groups such as Amnesty and Human Rights Watch have been limited by the deteriorating security situation.
Ms Choueiry believes an official body needs to be set up to ensure multi-national troops fulfil their mission while abiding by international humanitarian and human rights law.
"Whether the investigations are civilian or led by the judiciary, the most important thing is for them to be independent, impartial and transparent," she said.
But the effectiveness of such an organisation would be severely restricted by an order originally issued by the Coalition Provisional Authority, and renewed by the Iraqi government in 2004, that grants foreign forces immunity from Iraqi criminal and civil law.
Instead, the troops remain subject solely to the jurisdiction of their own states.
The US and UK have been accused of limiting the number and power of criminal prosecutions - in January, a US officer was punished with a reprimand and a $6,000 fine for killing a captured Iraqi general - or simply not undertaking them at all.
No prosecution was launched after a US marine was filmed shooting dead an incapacitated insurgent in a mosque in Falluja in November 2004.
Phil Shiner, a solicitor representing several Iraqi families taking the British government to court over human rights violations, told the BBC News website the small chance of anything being investigated effectively makes redundant the fourth Geneva Convention, which protects civilians in times of war or under occupation by a foreign power.
"The protection of the fourth Geneva Convention means nothing if the military does not investigate the crime," he said.
Mr Shiner has challenged the immunity of British troops in Iraq and their right to run their own investigations by arguing that European human rights law applied during their operations.
The UK High Court ruled in December that the British government would have to hold an "independent and effective" inquiry into the death of a man from Basra, Baha Mousa, because he died while in British custody.
Although the High Court also said it would be "premature" to conclude the British government was in breach of the European Convention on Human Rights before the outcome of the ministry's own investigation was known, such a ruling could have profound consequences for the armed forces.
It has considerably strengthened the case for the prosecution of soldiers found to have acted unlawfully.