Traces of bomb-grade uranium found two years ago in Iran came from contaminated Pakistani equipment and is not evidence of a clandestine nuclear weapons program, a group of US government experts and other international scientists has determined.

``The biggest smoking gun that everyone was waving is now eliminated with these conclusions,'' said a senior official who discussed the still-confidential findings.

Scientists from the United States, France, Japan, Britain and Russia met in secret during the past nine months to pore over data collected by inspectors from the International Atomic Energy Agency, according to US and foreign officials. Recently, the group, whose existence had not been previously reported, definitively matched samples of the highly enriched uranium - a key ingredient for a nuclear weapon - with centrifuge equipment turned over by the government of Pakistan.

Iran has long contended that the uranium traces were the result of contaminated equipment bought years ago from Pakistan. But the Bush administration had pointed to the material as evidence that Iran was making bomb-grade ingredients.

The conclusions will be shared with IAEA board members in a report due out the first week in September, according to US and European officials. The report ``will say the contamination issue is resolved,'' a Western diplomat said.

American officials have privately acknowledged for months that they were losing confidence that the uranium traces would turn out to be evidence of a nuclear weapons program. A recent US intelligence estimate found that Iran is further away from making bomb-grade uranium than previously thought.

The IAEA findings come as European efforts to negotiate with Iran on the future of its nuclear program have faltered and could complicate a renewed push by the Bush administration to increase pressure on Tehran.

Washington officials, eager to move the Iran issue to the UN Security Council - which has the authority to impose sanctions - have begun a new round of briefings for allies designed to convince them that Iran's real intention is to use its energy program as a cover for bomb building. The briefings will focus on the White House's belief that a country with as much oil as Iran would not need an energy program on the scale it is planning.

France, Britain and Germany have been trying for two years to convince Iran that it could avoid Security Council action if it gives up sensitive aspects of its nuclear energy program that could be diverted for weapons work. Iran has said it has no intention of making nuclear weapons and will not give up its right to nuclear energy.

Iran has offered to put the entire program under IAEA monitoring as a way of alleviating international concerns. But European and American officials have rejected that offer as it would still allow Iran access to bomb-making capabilities.

Iran built its nuclear program in secret over 18 years with the help of Abdul Qadeer Khan, a top Pakistani official and nuclear scientist who sold spare parts from his country's own weapons program to Iran, Libya and North Korea. Khan's black-market dealings were uncovered in 2003. He confessed on Pakistan television, was swiftly pardoned by President Pervez Musharraf, and is under house arrest.

Pakistan has denied IAEA inspectors access to Khan and to the country's nuclear facilities, but earlier this year it agreed to share data and some equipment with the inspectors to expedite the Iran investigation. Among the equipment were discarded centrifuge parts that match those Khan sold to Iran.

John Bolton, now US ambassador to the UN who served as the administration's point man on nuclear issues, suggested during congressional testimony in June 2004 that the Iranians were lying about the contamination.

The IAEA, in its third year of an investigation in Iran, has not found proof of a weapons program. But a few serious questions, some connected to Iran's involvement with Khan, remain unanswered.

While the investigation has been under way, Iran and the three European countries have been trying to reach a diplomatic accommodation. Their negotiations fell apart earlier this month and Iran resumed some nuclear work it had put on hold during the talks.