UNITED NATIONS -- U.N. officials said yesterday that they had intelligence information indicating that Iraq has tried to mislead the United Nations by understating its nuclear weapons program and the amount of weapons-grade nuclear material it possesses.
As a result, the special commission appointed by the Security Council to oversee the destruction or removal of Iraq's chemical and biological weapons and its nuclear weapons materials has ordered its experts to make new inspections of undisclosed nuclear sites in Iraq next week.
These inspections were ordered after the United States provided the commission with fresh intelligence from an Iraqi nuclear scientist who defected over a week ago to U.S. forces occupying an enclave in northern Iraq.
In Washington, government officials who follow the Iraqi nuclear program said the defector had said that Iraq had eight primary sites for nuclear research and development, three of which were bombed extensively during the U.S.-led campaign in the Persian Gulf war.
Most of the defector's reports remain unproved and will require additional research, the officials said.
U.N. officials said they were told that the Iraqi defector reported that Iraq had been trying to manufacture weapons-grade uranium using an old-fashioned technique called magnetic isotope separation, which the United States experimented with but abandoned while it was developing the world's first atomic bomb at the end of World War II.
A plant for this purpose was said to be situated in a secret underground site in the mountains not far from the northern Iraqi city of Mosul.
The defector also asserted that there were several other secret nuclear sites, officials here said.
A U.S. official, citing intelligence reports, said Iraq's nuclear weapons program had nevertheless been judged by analysts to have been more primitive than many feared.
Some of the eight sites may have been spared extensive bombing because they were not regarded as critical to Iraq's nuclear-weapons effort, an official said yesterday.
Still, he said, "it's a very major concern that we didn't find all the sites" for nuclear and chemical weapons research.
Iraq also removed an undetermined amount of equipment from its nuclear-weapons centers before the allied air campaign began, officials said, but they added that the action was not unexpected.
Under Security Council Resolution 687 setting out terms for lifting economic sanctions against Iraq, the Baghdad government is required to provide inventories of all its weapons-usable nuclear material as well as its chemical and biological weapons and its ballistic missiles to the special commission.
Iraq told the Security Council in April that it possessed only the 45 kilos, or nearly 100 pounds, of highly enriched uranium that it had declared to the International Atomic Energy Agency.
In Washington, Paul L. Leventhal, president of the Nuclear Control Institute, which studies nuclear proliferation issues, said administration officials had told him that the defector said Iraq had manufactured an additional 40 kilos -- about 88 pounds -- of weapons-grade material using magnetic isotope separation.
In a statement, the Nuclear Control Institute said J. Carson Mark, retired director of weapons design at the Los Alamos National Laboratory, has estimated that this amount would be sufficient for two atomic bombs of the kind dropped on Nagasaki, Japan, in 1945.
If confirmed, the defector's reports would highlight "ominous failures" both in U.S. intelligence-gathering and in international efforts to control nuclear proliferation, the Nuclear Control Institute said.
Iraq has signed the Nuclear Non-Proliferation Treaty, pledging not to acquire nuclear weapons and opening its declared nuclear sites to inspection by the Vienna-based Atomic Energy Agency.
In May, a team of agency inspectors sent by the special commission placed some of the 45 declared kilos of enriched uranium under tamper-proof seals at Iraq's Tuwaitha nuclear site but said the rest was buried under the rubble of a plant bombed by the allies.
At the same time, the inspectors visited a nuclear site at Tarmiya, north of Baghdad. Although they found nothing incriminating, officials say, the inspectors formed the impression that the plant had been stripped recently and hastily.
Members of the special commission have also expressed doubts about the accuracy of Baghdad's inventories of its chemical and biological weapons and its stocks of ballistic missiles.
U.N. officials said that under questioning from the commission, Baghdad recently admitted that it possessed substantially more chemical weapons than it had declared April 18.
At that time, it told the commission that it had about 11,000 shells, bombs and rockets filled with paralyzing nerve agents and poisonous gas as well as about 1,000 tons of chemicals used in the manufacture of such weapons.
In addition, Iraq acknowledged that it had more ballistic missiles than the 52 it reported April 18.