The Pentagon is close to ending its own inquiries into the deaths, initially attributed to a clash with militants but Iraq’s Prime Minister, Nouri Maliki, told Reuters news agency there was "a limit to the acceptable excuses" for civilian casualties.
The United State government is concerned that 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.
In a separate interview with the BBC Mr Maliki said that the civilians were "victims of a wrong operation. 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.
In Washington the White House promised that all details about the killings of civilians by US forces in Haditha would be made public when a military investigation is completed.
A leading US lawmaker, Democractic congressman John Murtha, and media reports have said the US Marines tried to cover up the killings.
White House spokesman Tony Snow said the US Marine Corps was taking an "active and aggressive role" in investigating the allegations made against them.
The US military is conducting a criminal investigation into the killings of Iraqi civilians by a Marines unit in Haditha, and a second one into whether an attempt was made to cover-up the deaths.
The White House spokesman said that President George W. Bush was concerned by the allegations. But Mr Snow said that the president was only briefed about the killings, by national security advisor Stephen Hadley, after a Time magazine report about Haditha in March.
Congressman Murtha, a harsh critic of the Iraq war, has accused the marines of seeking to "cover up" the killings. He has warned that the scandal could become more damaging for the United States than the Abu Ghraib prisoner abuse images which caused international controversy in 2004.