The United Nations' nuclear arms watchdog has begun conducting closed-door interviews with Iraq's atomic energy experts, marking a critical new stage in the U.N. effort to verify Baghdad's claims that it has destroyed its most lethal weapons of mass destruction, according to a spokesman for the agency.

Drawing from a list of hundreds of Iraqi officials linked to Iraq's former nuclear weapons program, officials from the International Atomic Energy Agency (IAEA) are seeking to determine whether Baghdad secretly began rebuilding that program after U.N. inspectors left the country in December 1998 on the eve of a U.S.-British bombing campaign.

While IAEA inspectors have routinely questioned Iraqi scientists at former nuclear weapons sites since they resumed inspections last month, this is the first time that they have asserted their right to conduct face-to-face interviews with individuals without the presence of an Iraqi government minder. It sets the U.N.'s nuclear sleuths ahead of their counterparts at the U.N. Monitoring Verification and Inspection Commission (UNMOVIC), who have yet to conduct confidential interviews with Iraq's biological, chemical weapons and ballistic missile experts.

"We are moving from an information-gathering phase to a more probing, investigative phase," the IAEA's chief spokesman, Mark Gwozdecki, said in a telephone interview from the agency's Vienna headquarters. "We can't talk about who, how or how many," he said of the scientists being questioned.

White House and State Department officials, meanwhile, dismissed an offer by Iraq this weekend to let CIA officials visit Iraq to participate in inspections and therefore, presumably, interviews. "It's nonsense," said one U.S. official. "The focus should be on Iraq coming clean."

But Defense Secretary Donald H. Rumsfeld would not rule out the possibility. "I don't know what the United States might consider doing," he said. "I suppose they invited intelligence people. And as I recall, I suppose the [intelligence] community is thinking about that at the present time."

The Bush administration has stepped up pressure on Mohammed ElBaradei, the Egyptian director general of the IAEA, and Hans Blix, the Swedish executive chairman of UNMOVIC, to speed the pace of inspections and to exercise their authority to question some Iraqi specialists outside the country, where they can speak freely without the fear of reprisals.

ElBaradei said in a recent interview that he would interview Iraqi scientists abroad if he received assurances from Washington that they could obtain political asylum or return safely to Iraq. "We are now in the process of interviewing people inside Iraq in private," ElBaradei added today in an interview with CNN. "But we are also working on the practical arrangements to take people out of Iraq."

Although Iraq's nuclear weapons program was largely destroyed by U.N. inspectors after the 1991 Persian Gulf War, the CIA and Britain's Joint Intelligence Committee believe that Baghdad has resumed its efforts, engaging in an intensive covert operation since 1998 to procure uranium and components that could be used in a nuclear weapons program. They have also raised concerns that Iraq has brought its nuclear weapons team back together.

"In the absence of inspections, however, most analysts assess that Iraq is reconstituting its nuclear program -- unraveling the IAEA's hard-earned accomplishments," according to a recent CIA report.

While the IAEA declined to name Iraqi specialists who have been questioned, officials said several individuals would be obvious subjects. Jaafar Dhia Jaafar, credited by U.N. specialists with heading up Iraq's covert nuclear weapons program, and Mahdi Obeidi, a uranium enrichment specialist, are central figures in Iraq's secret nuclear weapons program.

Jaafar was part of a senior Iraqi delegation that met numerous times with ElBaradei and Blix in New York and Vienna this year. Following one of those visits, Iraq's U.N. ambassador, Mohammed Douri, complained that the United States approached three members of the Iraqi delegation with an offer of political asylum. The offer was rejected, he said. But it remains unclear whether Jaafar was among those who had been contacted by the United States.

Pakistan, meanwhile, denied reports that the father of Pakistan's nuclear weapons program, Abdul Qadeer Khan, offered to help Iraq build a nuclear weapon in 1990. The Associated Press and the Times of London, citing U.N. documents, reported that an unidentified middleman, claiming to represent Khan, made the offer on the eve of the Gulf War. The IAEA maintained that Iraq never accepted the offer, according to the reports.

"We find it preposterous," said Mansoor Suhail, a spokesman for the Pakistani mission to the United Nations. "No responsible Pakistani scientist would enter into a a nuclear deal with any country."

Defense Secretary Donald H. Rumsfeld did not rule out the possibility of CIA officials participating in arms inspections, but another official called the Iraqi invitation "nonsense."