Equipment and materials that could be used to make nuclear arms have been vanishing in Iraq since the invasion, the United Nations has warned.
The US has been blocking full UN inspections in Iraq
Satellite images show entire buildings have been dismantled without any record being made, said Mohamed ElBaradei, the head of the UN nuclear watchdog.
Iraq's US-backed leaders have not reported to the UN on the state of nuclear plants despite a duty to do so.
But they have asked the UN to help sell off unwanted nuclear material.
Inspectors from Mr ElBaradei's International Atomic Energy Agency (IAEA), who established that Saddam Hussein had abandoned any nuclear weapons programme before the war, have not been allowed to move about Iraq freely by the US.
Apart from a couple of limited checks on the main nuclear facility at Tuwaitha last June after reports of looting - and with no teams now on the ground - the IAEA has to rely on satellite imagery and other sources.
In a letter to the UN Security Council, Mr ElBaradei said buildings related to Iraq's previous nuclear programme appeared to have been systematically dismantled and equipment and material removed.
"The disappearance of such equipment and materials may be of proliferation significance," the IAEA director general warned.
Sensitive technology such as rocket engines has turned up for sale abroad, Mr ElBaradei said.
However, high-precision "dual-use" items including milling machines and electron beam welders appear to have disappeared, as has material such as high-strength aluminium.
Mr ElBaradei called on any state with information on the location of such items to inform his agency.
The US removed nearly two tonnes of low-enriched uranium from Iraq earlier this year. The IAEA has verified that 550 tonnes of nuclear material still remain at Tuwaitha.
Iraq, the agency says, has asked for help to sell the nuclear material and in dismantling and decontaminating former nuclear facilities.
Mr ElBaradei reminded the Security Council that Iraq was still obliged to "declare semi-annually changes that have occurred or are foreseen at sites deemed relevant" by the IAEA.
However, since March 2003 "the agency has received no such notifications or declarations from any state", he said.
Last week, a report from chief US weapons inspector Charles Duelfer concluded that Saddam Hussein had stopped trying to build weapons of mass destruction following the 1991 Gulf War.