The International Atomic Energy Agency has published a confidential report urging Iran to allow it to inspect all its nuclear sites, Reuters reported.
The agency has named Isfahan as a site of concern over a new enrichment plant and weapons-grade uranium stored there.
The report was sent to IAEA members ahead of the quarterly meeting of its 35-member board next week, amid nuclear talks between the United States and Iran, the latest round of which concluded without a breakthrough on Thursday.
Like previous IAEA reports, it could be used by Washington to support its argument that Tehran has not been transparent about its nuclear activities - at a time when US President Donald Trump has built up forces in the region and is threatening new military action.
The United States and Israel bombed Iranian nuclear sites last June, and since then Tehran has refused to reveal what happened to its stockpile of highly enriched uranium or allow IAEA inspectors access to the sites where the enrichment took place.
"While the Agency acknowledged that the military attacks on Iran's nuclear facilities have created an unprecedented situation, it is crucial that it conducts verification activities in Iran without further delay," the report said.
Allowing inspections is "necessary and urgent", it added. It also noted that a successful outcome of the US-Iran talks would have a "positive impact on the effective implementation of safeguards in Iran and the resolution of the problems described in this report".
The IAEA estimates that Iran had 440.9 kg of uranium enriched to 60% before the Israeli-American attacks last year - enough, if further enriched, for 10 nuclear weapons. The agency and Western powers believe that most of it is still intact. Washington wants Tehran to give it up.
The report provides new details about the activity in Isfahan, where diplomats have said that much of the Islamic Republic's most highly enriched uranium is stored in a tunnel complex that appears to have prevented its destruction in June.
For the first time, the report confirmed that material enriched to 20% and 60% is stored there. In satellite images, the IAEA observed "regular vehicle activity around the entrance to the tunnel complex in Isfahan that stores uranium enriched to 20% and 60%", the IAEA said.
The US-Israeli attacks are believed to have destroyed or severely damaged the three uranium enrichment sites known to have been operating at the time. Shortly before Israel launched its attack, Iran said it was building a fourth enrichment plant in Isfahan, although the IAEA does not yet know its exact location or whether it is operational, the report said.
"A matter of growing concern is that Iran has never granted the Agency access to its declared fourth enrichment plant since it was first announced in June last year," the report said.