Iran has apparently been caught red-handed by inspectors who discovered sophisticated nuclear equipment that could be used to produce a bomb at an air force base outside Tehran.
The disclosure in a US newspaper prompted an angry denial from the Iranian government, which maintains that its nuclear research programme is entirely for civilian purposes.
But it also fuelled suspicions that Iran has continued to work on a nuclear weapons programme despite promising the foreign ministers of Britain, France and Germany last November that it was suspending all uranium enrichment.
Diplomats at the International Atomic Energy Agency (IAEA) said its inspectors had found components that could be used in advanced centrifuges for extracting enriched uranium. But the IAEA sources cautioned that this did not necessarily prove that Iran was working to build a nuclear weapon.
Iran is duty-bound to declare all nuclear-related activities to the IAEA. The fact that Tehran's last report contained omissions "obviously undermines its credibility", a diplomatic source said.
USA Today reported that the inspectors had last week found drawings of a P-2 gas centrifuge, which is much more advanced than a model Iran has admitted using to enrich uranium.
US officials yesterday said the parts were similar to those found in Libya, and may have been obtained through the black market route which flourished for more than a decade under Pakistan's top nuclear scientist Abdul Qadeer Khan.
The Bush administration is convinced that Iran is concealing a nuclear weapons programme from the UN inspectors who were instrumental in forcing Tehran to come clean about the scope of its nuclear activities.
But the Iranian Foreign Ministry spokesman, Hamid Reza Asefi, insisted: "In none of Iran's military centres is a nuclear programme being pursued and P-2 centrifuges do not exist in such centres."
Iran will be confronted with the latest finding at a governors' board meeting on 8 March of the IAEA, which will decide what action to take. President George Bush recently said it was unacceptable that a country under investigation should be on the nuclear agency's board - as is the case for Iran.
America may press the board to refer the issue to the UN security council, which has the power to impose sanctions. But so far they have been content to let Britain, France and Germany take the lead.