Iran has been criticised for its lack of co-operation by the UN's chief nuclear inspector at a meeting to decide on a united response.
Iran has been accused of keeping some of its nuclear activities secret [Photo: Digitalglobe]
"The way they have been engaging us on this issue has been less than satisfactory," Mohamed ElBaradei said.
Britain, France and Germany have put forward a draft resolution deploring Iran's conduct.
The International Atomic Energy Agency is investigating whether Iran is trying to develop nuclear weapons.
Tehran denies it is and says the agency should drop its investigation.
But Mr ElBaradei said he and his inspectors were unsure how far the Iranian nuclear programme went.
"We still have a central issue and that is whether Iran has
declared all of its enrichment activities," he said.
Enriched uranium can be used to make nuclear weapons.
He said there were still questions over the origins of traces of weapons-grade uranium discovered in the country, and the scale of its centrifuge-building programme.
Mr ElBaradei said there was no clear proof to back up Washington's claims that Iran is chasing nuclear weapons, but he called on Tehran to be "more transparent and proactive".
He said he would like the matter cleared up within "the next few months".
On Monday the IAEA began a meeting in Vienna to debate a toughly-worded resolution put forward by Britain, France and Germany.
The three countries have traditionally taken a softer line on Iran than the United States, which has called on the IAEA to report Tehran to the UN Security Council.
The European countries' resolution does not go that far, but would censure Iran while calling for further co-operation.
Last year, a deal was struck with Tehran under which Iran agreed to suspend enrichment activities and accept more intrusive inspections of its nuclear sites in exchange for technology.
Bethany Bell, the BBC's correspondent in Vienna, says the IAEA's toughened stance puts that deal under pressure.
The draft calls for Iran to freeze additional parts of its nuclear programme, something Iran rejects.
The Iranian Foreign Minister, Kamal Kharrazi, says Iran has to be recognised by the international community as a member of the nuclear club.
Iran says its nuclear activities are purely peaceful and aimed solely at generating electricity.
Iranian MPs have threatened to retaliate if the IAEA pushes them too hard.
"If Western governments impose extreme demands the
parliament will not sign the protocol," said parliamentarian
Mohammad Reza Tajeddini on Monday, referring to a UN protocol on snap nuclear checks.