The U.N.'s nuclear watchdog agency reported Monday that massive quantities of high explosives at an Iraqi weapons facility have disappeared, including some material under U.N. seal because of its potential use to detonate a nuclear bomb.

U.N. and Iraqi officials indicated the explosives were lost while the country was under U.S. occupation. But U.S. officials suggested that the munitions may have disappeared before the U.S.-led forces established full control over the country. They said a search of the facility by U.S. troops shortly after the fall of Baghdad last year turned up no evidence of the explosives.

The conflicting accounts generated confusion over who bore responsibility for allowing the loss of the explosives that had been stored at the sprawling Qaqaa weapons facility outside Baghdad before the war. The munitions included the explosives RDX, PETN and HMX.

HMX, the only one of the three under U.N. seal, has application as a nuclear trigger. But it and the other two can also be used to demolish buildings, down jetliners and produce warheads for missiles.

The disappearance of the material raised the possibility that some could already have found its way into explosive devices used against U.S. and allied troops in Iraq -- or could do so in the future. But U.S. officials in both Washington and Baghdad played down such a likelihood, noting that the car bombs and roadside explosives that have menaced troops have tended to be made from old artillery shells or dynamite.

Pentagon and State Department officials said the matter has been referred to the Iraq Survey Group, a CIA-run agency charged with accounting for what became of Iraq's unconventional weapons programs. But Charles A. Duelfer, a former U.N. weapons inspector who heads the group, said in a telephone interview that he had received no such orders.

Duelfer also said that a U.S. team inspecting the site in May 2003 turned up no evidence of explosives under U.N. seal. "My sense is, it's been looted, it's gone missing," he said of the material. "I don't know the specifics, but it's not there now.

Mohamed ElBaradei, director general of the International Atomic Energy Agency, presented the U.N. Security Council with an Oct. 10 letter from the interim Iraqi government reporting the loss of the explosives. Word of the letter was reported Monday by the New York Times and CBS News.

The disclosure came less than a month after ElBaradei warned the council that U.N. satellite photos had detected "widespread and apparently systematic dismantlement" of buildings linked to Iraq's former covert nuclear weapons program. Those buildings "housed high precision equipment" that had been subject to U.N. monitoring during Saddam Hussein's reign.

The letter from Mohammad J. Abbas, a senior official in the Iraqi Ministry of Science and Technology, said that nearly 215 tons of HMX, 156 tons of RDX and 6 tons of PETN had gone missing after April 9, 2003, the day Baghdad fell to U.S. forces. The letter blamed a "lack of security" for the loss.

The IAEA first discovered Iraq's stores of HMX and RDX in 1991, after the Persian Gulf War. The agency placed more than 250 tons of HMX under U.N. seal at Qaqaa. An additional ton of the explosive was housed at the nearby facility of Hatteen, about 24 miles south of Baghdad.

The IAEA authorized U.N. weapons inspectors to use some of the explosives in 1996 to destroy an Iraqi biological weapons facility. More than 32 tons of HMX under IAEA seal disappeared from the site from 1998 to 2002. Iraq told U.N. authorities that it had used the explosive in its cement industry.

ElBaradei informed the Security Council on Monday that the last time IAEA inspectors were able to verify the presence of the explosives at Qaqaa was in January 2003, two months before the U.S. invasion began.

Administration officials have acknowledged the inability of U.S. troops to secure the large stocks of weaponry discovered after the invasion. But officials at both the Pentagon and State Department suggested Monday that the high explosives said to have vanished from Qaqaa may have been removed from the facility by Hussein loyalists before the invasion, then hidden or sent abroad.

"We don't know that this site was looted," a senior State Department official said, speaking on the condition of anonymity citing department policy. "All we know is that it's not there. We don't know whether it was moved by Saddam before the war."

The official added that U.S. authorities lacked a complete list of material that had been placed under IAEA seal. "We weren't doing accounting of everything under IAEA seal. Some stuff yes, but not all of it," he said.

Pentagon spokesman Bryan Whitman said U.S.-led forces searched the Qaqaa facility after the invasion.

"Coalition forces were present in the vicinity at various times during and after major combat operations," he said. "The forces searched 32 bunkers and 87 other buildings at the facility. While some explosive material was discovered, none of it carried IAEA seals."

In satellite photos of the Qaqaa site taken in November 2003 and shown to The Washington Post on Monday by senior U.N. officials, signs of damage from previous U.S. bombing campaigns and looting were evident. But the facilities that stored HMX and RDX were still largely intact, according to the officials.

Graham reported from Washington. Staff writers Walter Pincus and Robin Wright contributed to this report from Washington.