Mohamed ElBaradei, director-general of the International Atomic Energy Agency, said he received assurances from Washington that the Tuwaitha nuclear research center would be protected and that access to the complex would be restricted.
"Until our inspectors return to Iraq, the U.S. has responsibility for maintaining security at this important storage facility," he said in a statement. "As soon as circumstances permit, the IAEA should return to verify that there been no diversion of this material."
Separately, the Los Angeles Times reported that 2,500 barrels of uranium that could be used to produce nuclear weapons had been left unguarded at the site for several days following the withdrawal of Iraqi troops.
The newspaper said Marine combat engineers secured the uranium on Wednesday after the State Department warned the military of the danger.
ElBaradei did not directly respond to unconfirmed reports that radiation may be leaking from the complex 18 miles south of Baghdad.
But he made it clear that the site is well-known to the nuclear agency, suggesting there was little to document the notion that Saddam Hussein was actively pursuing nuclear weapons there or that the Marines had uncovered anything new.
High radiation levels are normal at the site, "and great care must be taken" when entering storage buildings, the IAEA said Friday.
Leaders of the Marine combat engineering unit reported finding an underground network of laboratories, warehouses and bombproof offices beneath the 70-building complex.
The Marines said 14 buildings at the site emitted unusually high levels of radiation, and that a search of one building revealed numerous drums containing highly radioactive material.
However, the Vienna-based IAEA — which has inspected Tuwaitha at least two dozen times and maintains a thick dossier on the site — said Iraq was allowed to keep several tons of low-grade uranium and other nuclear material there under IAEA seal because the material could not be used directly for weapons.
An expert familiar with the inspections, speaking on condition of anonymity, told The Associated Press on Friday that some of those seals apparently were broken before the Marines arrived at Tuwaitha. It was unclear whether any materials were missing.
Tuwaitha contains 1.8 tons of low-grade enriched uranium and several tons of natural and depleted uranium. The uranium was inspected by the U.N. nuclear agency twice a year.
IAEA inspectors visited Tuwaitha about a dozen times since December and most recently on Feb. 6. It was among the first sites that IAEA inspectors sought out after the resumption of inspections on Nov. 27 after a nearly four-year break.
On at least one occasion, inspectors with special mountaineering training went underground there to have a look around, according to IAEA documents.
American intelligence analysts said before the U.S.-led military campaign began that new structures photographed at Tuwaitha might indicate a revival of weapons work. IAEA inspectors checked but found nothing.
The Tuwaitha complex, run by the Iraqi Atomic Energy Commission on a bend in the Tigris River, was the heart of Saddam's former nuclear program and was involved in the final design of a nuclear bomb before Iraq's nuclear program was destroyed by U.N. teams after the 1991 war.
The IAEA, charged with the hunt for evidence of a nuclear program in Iraq, told the Security Council just before the war that it had uncovered no firm evidence that Saddam was renewing efforts to add nuclear weapons to his arsenal.