The UN Monitoring, Verification and Inspection Commission (UNMOVIC), created to track Iraq's weapons of mass destruction, based its conclusion in a Friday report on satellite imagery from sites with material that had weapons potential.
Repairs and new construction have begun at 10 of the 90 sites, said the report to the UN Security Council from Demetrius Perricos, the chief weapons inspector.
UNMOVIC, using photographs and serial numbers, previously reported that the looting of unguarded sites resulted in missile engines turning up among scrap in the Dutch port Rotterdam as well as in Jordan.
Destroyed in the bombing
But the new report gave more comprehensive figures on how many of the unguarded sites were looted or destroyed in bombing during the US invasion of Iraq two years ago.
Before they left Iraq, UN inspectors had examined 411 sites, the report said. After the war, they examined 353 sites and determined that 70 of them were "subjected to varying degrees of bomb damage."
"The continuing examination of the imagery has revealed that approximately 90 of the total 353 sites analysed containing material of relevance have been stripped and/or razed," Perricos said in the report.
UN inspectors were in Iraq between 1991 and 1998 searching for and destroying nuclear, biological and chemical arms and materials as well as long-range missiles after the first US-led Gulf war that drove Baghdad's troops from Kuwait.
"The continuing examination of the imagery has revealed
that approximately 90 of the total 353 sites analysed containing material of relevance have been stripped and/or razed"
Saddam Hussein's government let the UN inspectors back in late 2002 after a US war threat. But the United States refused to allow them to return after the March 2003 invasion.
The Security Council will discuss the report on Tuesday amid continuing questions over the future of the agency.
Council members since the end of the war have pressed American and British officials to utilise UNMOVIC's vast research and allow the inspectors to complete monitoring work.
The United States said it did not want to take up the issue until its own searches ended, first led by David Kaye, a former International Atomic Energy Agency official, and then Charles Duelfer, also a former UN inspector. But there are indications Washington now wants to discuss closing down UNMOVIC.
Friday's report also said the UN inspectors agreed with Duelfer, whose CIA-organised Iraq Survey group had expressed concern about biological materials that were unaccounted for since 1991.
Duelfer's report in October said his inspection group found no weapons of mass destruction in Iraq, President George Bush's main reason for the invasion.