A report by the International Atomic Energy Agency is said to conclude that Iran has failed to meet its obligations under the nuclear non-proliferation treaty.
The IAEA's report, based on its inspections of Iranian nuclear facilities, is not officially due out until 16 June. But it has been heavily leaked in advance.
According to detailed accounts, it concludes that Iran has failed to meet its obligations to report the acquisition and processing of nuclear material and the facilities involved.
But it did say that Iran was taking action to rectify these failures.
The report on Iran's nuclear facilities has been leaked
However, the report noted that the agency would not be able to provide credible assurances about Iran's nuclear activities unless Tehran signed an additional protocol which it has so far refused to do.
The Iranian foreign ministry spokesman, Hamidreza Asefi, said that Iran had done nothing which would violate its commitments under the non-proliferation treaty and it had answers, he said, to all the questions being raised.
While the report uses the word failure, rather than violation, American officials have already described it as deeply troubling.
Washington is putting strong pressure on Russia to stop its involvement in Iran's nuclear energy programme.
The Russians have so far resisted, but they have said that they too would like Tehran to open up more fully to international scrutiny.
So the IAEA's report is likely to increase the pressure on Iran to sign that additional protocol which would mean tougher and more conclusive inspections.