The United Nations atomic agency says questions about Iran's nuclear programme remain unanswered, despite an intensive investigation.
The EU and the US have hardened their stance against Tehran
In a key report, the IAEA urges Iran to clear up outstanding issues, saying Iran's full transparency was "overdue".
Tehran said the findings were politically motivated but that it would continue to co-operate with the agency.
The US has accused Iran of seeking to develop nuclear weapons, but Iran says its programme is for peaceful purposes.
Iran resumed uranium conversion activities at its Isfahan plant last month after a 10-month suspension.
The report, written by IAEA chief Mohamed ElBaradei, says about four tons of yellowcake - uranium ore - had been fed into the conversion process at Isfahan.
However, the report said Iran has not engaged in uranium enrichment, which if processed to a high grade, can be used in nuclear weapons.
But the IAEA was concerned that after two and a half years of "intensive inspections" questions remained about Iran's nuclear programme.
The report said the agency was "still not in a position to conclude that there are no undeclared nuclear materials or activities in Iran".
It said Iran's full transparency was "indispensable and overdue".
Outstanding issues included traces of enriched uranium contamination found at various locations in Iran and its research into advanced centrifuge technology.
Iran's chief nuclear negotiator Ali Larijani said many of the questions raised in the report have already been answered.
"There are some minor questions remaining and our co-operation with the agency will continue in order to answer those questions," he told Iran state TV said.
The EU has indicated it is ready to take steps towards referring Iran to the UN Security Council if the suspension was not resumed.
Tehran resumed its nuclear fuel work after rejecting incentives tabled by the UK, France and Germany aimed at persuading it to abandon nuclear fuel production.