The Tuwaitha nuclear facility is of particular concern to inspectors
The UN nuclear watchdog agency has urged Washington to allow it to investigate nuclear sites in Iraq that have reportedly been looted.
Mohamed ElBaradei - head of the International Atomic Energy Agency - wrote to Washington last Wednesday to request that an investigative team be allowed into Iraq, but has not yet received a response, according to a spokeswoman.
The agency is concerned that radioactive material known to be stored at several Iraqi sites could pose health and environmental risks, and there are also fears they could be used to create a so-called "dirty bomb".
"We have been assured by the US that they would secure these facilities, but the agency finds these reports [of looting] disturbing," said IAEA spokeswoman Melissa Fleming.
She told BBC News Online that the alleged looting at the Tuwaitha site - a large, sprawling facility about 75 kilometres south of Baghdad - was of particular concern.
Two reports over the weekend gave rise to concern:
On Saturday, a Washington Post reporter travelling with a special US defence department team visited the Baghdad Nuclear Research Facility. US soldiers at the site told him Iraqis had been "coming in by the score" for two weeks. The team found radioactive material scattered around the site.
Also on Saturday, a New York Times reporter with the same team visited the nearby Tuwaitha site, again finding radioactive material stored haphazardly around the site and indications that, even by Saturday, little or nothing had been done to prevent looting.
But on Monday, a State Department spokesman played down the Washington Post report, saying that none of the material involved was suitable for making nuclear weapons.
"Coalition forces have secured the facilities that house the natural and low enriched uranium that was at those sites," the spokesman, Richard Boucher, said.
"All of this uranium would require significant processing in order to be suitable for enrichment for weapons use."
Mr Boucher said Washington was constantly in touch with IAEA on various issues, but added that "there's no decisions at this point about what role they may or may not play in terms of evaluating and monitoring."
The IAEA urgently recommended both the Baghdad and Tuwaitha facilities be given protection on 11 April.
It appears US soldiers were sent to some sites but were unwilling or unable to seal them off.
UN chemical and nuclear weapons inspectors - whose relations with the US were soured over abandoned weapons inspections in Iraq - have not so far been allowed to return to Iraq.
But the IAEA is said to have a detailed inventory of radioactive materials at several sites around the country which would help in identifying if such materials have indeed been taken.