Satellite imagery shows entire buildings in Iraq have been dismantled.
They once housed high-precision equipment that could produce nuclear bombs, the International Atomic Energy Agency (IAEA) said in a report to the UN Security Council on Monday.
Equipment and materials helpful in making bombs have also been removed from open storage areas in Iraq and have disappeared without a trace, according to the satellite pictures, IAEA Director-General Muhammad al-Baradai said.
While some military goods that disappeared from Iraq after the March 2003 US-led invasion - including missile engines - later turned up in scrap yards in the Middle East and Europe, none of the equipment or materials known to the IAEA as potentially useful in making nuclear bombs has turned up yet, al-Baradai said.
The US barred the return of UN weapons investigators after launching the war on Iraq in March 2003, preventing the IAEA from keeping tabs on hi-tech equipment and materials up to the present day.
Under anti-proliferation agreements, the US authorities who administered Iraq until June, and the Iraqi interim government that took over power, would have to inform the IAEA if they moved or exported any of that material or equipment.
The US barred UN weapons
inspectors after March 2003
But no such reports have been received since the invasion, officials of the watchdog agency said.
The US has not publicly commented on earlier UN reports disclosing the dismantling of a range of key weapons-making sites, raising the question of whether it was unable to monitor the sites.
Nuclear bomb risk
In the absence of any US or Iraqi accounting, council diplomats said the satellite images could mean the gear had been moved to new sites inside Iraq or stolen.
If stolen, it could end up in the hands of governments or groups seeking nuclear weapons.
Military bases have been looted
for scrap metal
"We simply don't know, although we are trying to get the information," said one council diplomat, speaking on condition of anonymity.
US officials had no immediate comment on the report.
US President George Bush justified the invasion of Iraq by saying the oil-rich country was on the brink of developing nuclear, biological and chemical weapons that might be used against the United States.
US intelligence services have since debunked pre-war intelligence on Iraq and said the country had no illicit weapons programme of any kind after 1991.
A new CIA report by chief US weapons investigator Charles Duelfer made clear last week former Iraqi president Saddam Hussein had all but given up on his nuclear programme after the first Gulf war in 1991.
Al-Baradai, whose agency dismantled Iraq's nuclear arms programme over a decade ago, had drawn similar conclusions to the Duelfer report well before the March 2003 invasion.
In his latest report, al-Baradai said the IAEA remained "concerned about the widespread and apparently systematic dismantlement" of former nuclear sites it used to monitor.
"As the disappearance of such equipment and materials may be of proliferation significance, any state that has information about the location of such items should provide IAEA with that information," his report said.