Iraq's interim government has invited the UN nuclear watchdog to check on the disappearance of materials from its former nuclear sites.
The US has been blocking full UN inspections in Iraq
Iraqi Technology Minister Rashad Omar said International Atomic Energy Agency (IAEA) inspectors had free access and could come back when they wanted.
He said there had been looting at the start of the US-led invasion, but the sites were now secure.
An IAEA report says technology is missing from Iraq's nuclear sites.
The agency says satellite imagery shows that entire buildings have been dismantled, while materials and specialised equipment have disappeared.
The IAEA responded to Iraq's invitation by saying that any decision on the return of its inspectors would have to come from the UN Security Council.
IAEA spokeswoman Melissa Fleming said the agency was concerned that sensitive technology might have fallen into the hands of those involved in the black market in nuclear weapons.
She said scrap metal from Iraqi nuclear sites, some of which was mildly radioactive, had been turning up abroad.
US State Department spokesman Richard Boucher echoed the IAEA's concerns, saying Washington had no detailed knowledge of what might have disappeared or where it might have gone.
"That's a problem that occurred right after the war that we do think has been brought under control," he said.
However, Mr Boucher added that the IAEA had been allowed access to Iraq's main nuclear site, Tuwaitha, twice since the US-led war - in June last year and August this year.
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 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.
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.
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, IAEA head Mohamed 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.