US officials say there may be as many as 260 mass graves in Iraq, containing the bodies of at least 300,000 people.
Relatives may be destroying evidence
Forty sites had already been confirmed, said Sandra Hodgkinson, who heads the coalition's mass grave action plan.
Ms Hodgkinson was addressing a conference in Baghdad to prepare Iraqi officials for disinterring mass graves.
The mass graves mostly included the remains of ethnic Kurds and Shia Muslims killed for opposing the regime between 1983 and 1991, she said.
Although a number of graves have been discovered, there are fears relatives may be destroying evidence as they try to recover their loved ones.
The three-day conference aims to prepare Iraqi human rights workers and officials for the process of disinterring graves and convincing families not to dig up bodies themselves.
"We believe, based on what Iraqis have reported to us, that there are 300,000 dead and that's the lower end of the estimates," she said.
"We have found mass graves of women and children, with bullet holes in their heads and we have found mass graves of husbands and fathers out in the desert where they were buried," Mr Hodgkinson told the conference.
1983: attacks on Kurds
1986-1988: chemical weapons attacks on Kurds
1991: crushing of a southern Shia revolt
1991: crushing of Kurdish insurrection
"We met survivors who crawled out of mass graves after being buried alive. We met with families whose loved ones did not escape."
Ms Hodgkinson said the task of identifying bodies and preparing evidence for tribunals could take years and millions of dollars.
Teams of foreign forensic experts were expected to start working on mass grave sites in Iraq in early January.
It could take years to complete the search, disinterrement and reburial
Between eight and 20 sites have been selected, she said.
But she pleaded for patience from the relatives.
"Mass graves provide many answers," Ms Hodgkinson said.
"They tell the story of missed loved ones. They allow families to regain remains to be buried in dignity. They are the first step to reconciliation."
Iraqi Minister for Human Rights, Abdel Bassit Turki, said it was urgent to protect the mass graves and prevent people and families from disturbing them.
Mr Turki added little of the $100m promised at last month's donor conference in Madrid had so far materialised.