The head of the United Nations nuclear watchdog said today that his inspectors were unsure if Iran had declared all its activities connected with the enrichment of uranium.
"We still have a central issue and that is whether Iran has declared all of its enrichment activities," Mr Mohamed ElBaradei, chief of the International Atomic Energy Agency (IAEA), said before a key IAEA governing board meeting on Iran.
Mr ElBaradei said there were two questions related to Tehran's enrichment programme that the IAEA had been unable to answer - the scale of Iran's advanced P-2 centrifuge programme and the origin of traces of bomb-grade uranium found in the country. Uranium can be used to make atom bombs.
Last week the IAEA said there were signs Iran's P-2 programme was planned on a massive scale, not as a tiny "research and development" project as the Iranians insist.
A private Iranian company had discussed the purchase of parts for enough centrifuges to make material for several nuclear bombs per year.
Mr ElBaradei, who has said there is no clear proof Iran is pursuing nuclear weapons as Washington alleges, said Tehran was not co-operating enough with UN inspectors.
"The way they (Iran) have been engaging us on this issue has been less than satisfactory," he said, adding that Iran needed to be "more transparent and proactive".
Iran, which insists it has no weapons programme, has dismissed the questions of the uranium traces and P-2's as "minor issues" and has called for an end to the UN investigation.