Iraq's interim government is playing down concerns over the disappearance of materials from nuclear sites that could be used to make atomic weapons.
The US has been blocking full UN inspections in Iraq
The UN nuclear monitoring agency says satellite imagery shows that entire buildings have been dismantled and specialised equipment is missing.
Interim Technology Minister Rashid Omar said equipment was taken by looters soon after the US-led invasion.
But since then, he told the BBC, Iraq's nuclear facilities had been secured.
An International Atomic Energy Agency (IAEA) spokesman, Mark Gwozdecky, said the Americans themselves might have been responsible for dismantling buildings and removing materials, but had not yet responded to requests for clarification.
He added that the problem would remain one of nuclear proliferation until it could be shown that the missing materials were in responsible hands.
British Foreign Secretary Jack Straw has ordered a detailed report into the matter.
Mr Omar said he was not aware of any buildings being demolished at Iraq's main nuclear site at Tuwaitha.
But he added that eight buildings there were being rehabilitated as part of a plan to turn the site into a science and technology park for peaceful research.
"As far as I am concerned, the ministry of Science and Technology which controlled the Tuwaitha site, which included the Iraqi nuclear facilities, the location was looted - the buildings, the equipment - immediately after the collapse of the regime," he told the BBC.
"Then afterwards it came under the control of the coalition forces and the area was well-protected until the transition of sovereignty.
"After the transition of sovereignty to us it is under our control and the location is well-protected and there is no looting."
Mr Omar insisted that Iraq would fulfil its responsibilities to the IAEA, and inform it of any equipment being moved.
He invited the agency to come when it wanted to Iraq, promising free access.
Inspectors from the 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 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.
'On sale abroad'
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 had 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.