By Mark Huband, Security Correspondent, in London
Nuclear inspectors have cast doubt on Iran's claim that components found at sites in the country were contaminated by enriched uranium before being imported, raising the suspicion that Iran's enrichment programme may have been more widespread than it previously admitted.
The inspectors from the International Atomic Energy Agency (IAEA), the United Nations nuclear watchdog, have also questioned why Iran had placed large orders to import magnets for use in centrifuges that could enrich uranium when it had previously said the components were produced at a factory in Iran.
The concerns are expressed in a confidential IAEA report which will be discussed by its governors on June 14.
The US has accused Iran of developing a nuclear weapons programme, though Tehran has insisted its programme is for civil use.
Last October, Iran agreed to suspend all uranium enrichment and open its facilities to intrusive inspections by the IAEA.
During the inspections, IAEA officials found traces of high- and low-enriched uranium contamination at several sites. Iran initially said it had been caused by contamination in the country - widely acknowledged to be Pakistan - from which it said it had obtained the components.
However, a diplomat close to the inspections said that this explanation was increasingly difficult to believe. The diplomat said the contamination was concentrated "in clusters on domestically-produced equipment" and that the levels of contamination were uneven.
"Had the equipment been imported, there would have been even contamination," the diplomat said.
The IAEA is considering the possibility that Iran either imported highly enriched uranium, or produced it domestically.
The report says the information provided by Iran on this issue is unlikely to "contribute further to the resolution of the contamination issue unless more information becomes available about the origin of the components".
Pakistan, from where Iran has said it received assistance in its nuclear programme through the activities of the scientist AQ Khan, has refused to allow the IAEA to seek to match the uranium traces found in Iran with uranium produced in Pakistan.
Until these tests are carried out, the suspicion will remain that Iran enriched the uranium itself.
The report also raises the issue of whether Iran had planned to develop a much larger nuclear enrichment programme than was first thought. It states that Iranian officials now say they had ordered 4,000 magnets for use in the centrifuges for uranium enrichment, and had planned to buy at least 4,000 more through a European intermediary.
Iran originally said its centrifuges were used in research, but the scale of the orders has undermined this claim, diplomats say.
"It is of concern. If you're ordering 4,000 magnets it suggests an active programme," said a diplomat.
The IAEA has now asked Iran to explain how such large-scale procurement fits with the small-scale research programme Tehran previously said it was carrying out.