American troops and contractors caused substantial damage to the archaelogical site at Babylon in Iraq after the 2003 invasion, a new UN report says.
The report says key structures were harmed and the site was subjected to "digging, cutting and levelling".
But UN cultural officials stress the damage did not begin when the Americans arrived, or end when they left.
The US says looting while Babylon was under their control would have been worse had they not been there at all.
The new report was issued on Thursday in Paris by the UN's cultural agency, UNESCO.
It comes after five years of investigations by Iraqi and international academics, some of whom have previously been critical of damage caused when US troops were based at Babylon in 2003 and 2004.
The 4,000-year-old city south of Baghdad was once home to the Hanging Gardens of Babylon, one of the seven wonders of the ancient world.
The UNESCO report says troops and contractors dug long trenches through the ancient ruins, bulldozed hilltops and drove heavy vehicles over the fragile paving of pathways which were once held sacred.
Experts say that, among the most famous structures harmed, were the city's Ishtar Gate and a main processional street.
Archaeologist John Curtis, of the British Museum - who has visited Babylon, said:
"There has indeed been a considerable amount of damage."
He said dragon carvings from the Ishtar Gate seemed to have been vandalised by looters while the area was under the control of American forces.
Many of Babylon's famous artifacts were plundered by European archaelogists during the 19th century and are on display at some of the world's best-known museums.
Looting and trading on the black market have continued since the site was handed back to Iraqi control in late 2004, UNESCO says.
The organisation has pledged to move towards naming Babylon a World Heritage Site - though officials say the extent of the damage means it is too early to assess how much restoration and protection work will cost.
UNESCO previously declined to list Babylon as a World Heritage Site on the grounds that restoration and rebuilding work carried out there under the regime of the former Iraqi leader Saddam Hussein had badly distorted the original ruins.