Iraq will investigate allegations that US marines carried out a massacre of civilians in Haditha in November, the country's prime minister has said.
The official US version differs widely from that of locals and the media
Nouri Maliki told Reuters news agency there was "a limit to the acceptable excuses" for civilian casualties.
The Pentagon is close to ending its own inquiries into the deaths, initially attributed to a clash with militants.
Observers say the incident could deal a more serious blow to US standing than the Abu Ghraib scandal.
According to initial US military reports, 15 civilians and eight insurgents died after a bomb killed a marine in Haditha, a militant stronghold in Anbar Province.
The civilians were "victims of a wrong operation", Mr Maliki said in a separate interview with the BBC.
"It is not justifiable that a family is killed because someone is fighting terrorists."
Speaking to Reuters, he said his government was worried by "the increase in 'mistakes'" and would ask "for answers not only about Haditha but about any operation... in which killing happened by mistake".
"We will hold those who did it responsible," he added.
The BBC's Justin Webb reports from Washington that enough material has now been leaked to the US media about events in Haditha to suggest to many Americans that allegations of a massacre are very serious and may well be true.
US investigators are looking at both the actual events in Haditha on 19 November and an alleged cover-up by troops.
Haditha has seen regular deadly attacks on US troops
The military said at the time that the civilians were killed as a result of either the bomb or a gun battle which erupted afterwards, in which the militants were reportedly killed.
But reports from Iraqi witnesses and in the US media allege that marines went on a rampage.
According to the Wall St Journal, there is evidence that marines killed civilians, including women and children, without provocation.
Several marines are likely to be charged with murder and others with attempting to cover up the incident, the newspaper said, quoting civilian and military officials close to the investigations.
One of the marines in Haditha that day, Lance Cpl Roel Ryan Briones of Hanford, California, told the Los Angeles Times he had taken photos and carried bodies out of homes as part of a clean-up crew:
"They ranged from little babies to adult males and females. I'll never be able to get that out of my head. I can still smell the blood."
John Murtha, a Democratic Congressman and former marine, has said he believes civilians in Haditha were murdered and the incident was covered up.
"They killed innocent civilians in cold blood and that's what the report is going to tell," he said.
"It is as bad as Abu Ghraib, if not worse," he told CNN television.
Gen Peter Pace, the chairman of the Joint Chiefs of Staff and chief military adviser to the US president, said on Monday it would be premature for him to judge the outcome of the investigations.
"If the allegations as they are being portrayed in the newspapers turn out to be valid, then of course there'll be charges," he said.
"We'll get to the bottom of the investigation and take the appropriate action."