The UN nuclear watchdog found uranium particles at two Iranian sites it inspected after months of stonewalling.
It is a discovery that could complicate US efforts to revive nuclear diplomacy, Reuters reported on Friday
Although the sites where the material was found are believed to have been inactive for nearly two decades, opponents of the nuclear deal say evidence of undeclared nuclear activities shows that Iran has not been acting in good faith.
Iran's ambassador to the International Atomic Energy Agency, Kazem Gharibabadi, declined to comment, as did the agency itself.
A senior Iranian official said, "We have nothing to hide. That is why we allowed the inspectors to visit those sites."
Iran had informed the international body of its plans to install more of its advanced IR-2m centrifuges at an underground uranium enrichment plant at Natanz, according to news reports.
An agency report stated the plans would deepen a breach of Iran’s nuclear deal.
Iran has set a deadline of next week for US President Biden to lift sanctions re-imposed by his predecessor Donald Trump or it will halt snap agency inspections.
Next week is also when the international body is expected to issue a quarterly report on Iran's nuclear activities.
The IAEA chairman, Rafael Grossi, said he would meet Iranian officials on Saturday to find a "mutually agreeable solution, compatible with Iranian law" so that the agency could continue inspections.
Iran was warned on Thursday by ministers at the G7 virtual meeting that it was "playing with fire" by escalating its nuclear activity.
The IAEA's full findings are a closely guarded secret and only a small number of countries have been informed of the specifics.
Five diplomats said that after the agency confronted Iran with the findings, officials gave unsatisfactory answers.
Two diplomats said Iran told the agency the traces were the result of contamination from radioactive equipment moved there from another site, though upon inspection, it was found that the particles at the sites did not match.
One diplomat briefed on the exchanges but not the detailed findings said Iran had given "implausible answers", describing Iran's response as "typical delaying tactics".
The agency has said it suspects one of the sites hosted uranium conversion work, a step in processing the material before enrichment, and the other was used for explosive testing.