The head of the UN's nuclear watchdog says he is "seriously concerned" about omissions in Iran's declaration on its nuclear programme submitted last year.
Iran says the world must accept its nuclear status
Mohammed ElBaradei was speaking as the International Atomic Energy Agency met to consider how to proceed with Iran.
But Iran's ambassador said Tehran had never said the dossier was complete.
His US counterpart said Iran had changed its "stories to fit the facts" - and suggested the IAEA would be dealing with Iran for many years.
The 35-nation board of governors is discussing a critical report on Iran, which notes that Tehran failed to reveal sensitive research in a declaration submitted last October.
Iran had violated the nuclear Non-Proliferation Treaty (NPT) for many years, Mr ElBaradei said at the Vienna talks.
Mr ElBaradei singled out for particular concern Iran's failure to declare that it was researching advanced centrifuge designs, known as P2, capable of producing highly enriched uranium.
The IAEA has persuaded Iran to permit tougher checks
This, he said, had been "a setback to Iran's stated policy of transparency".
Speaking to reporters on the sidelines of the Vienna meeting, Iran's ambassador said their declaration was never intended to be a full picture of their atomic past.
They had been the victim of a "war of propaganda" and "misquoted" as saying the declaration was complete, Pirooz Hossein said.
Last October, Iran said the declaration "fully disclosed" its "past peaceful activities in the nuclear field".
But the US ambassador to the IAEA said the Iranians changed their stories each time IAEA inspectors found something that had not been declared to them.
"This [IAEA] board has a lot more work to do and I expect we'll be dealing with the Iran issue for many years to come," Kenneth Brill said.
Mr ElBaradei rejected Iran's demands that the IAEA close its files on the country's nuclear programme and accept that it is a peaceful project.
The matter would be closed, he said, once the IAEA had completed its work on verifying Iran's past activities.
Iran halted its enrichment programme last year under international pressure but has indicated the move is only temporary.
European states led by Germany, France and the UK have favoured a more conciliatory approach to Iran, pointing to the complicated political situation within the Islamic republic.
The IAEA board meeting is also hearing a report on the dismantling of Libya's nuclear weapons programme.
Mr ElBaradei also criticised Tripoli for past violations of the NPT and urged similar transparency and openness from them. But the IAEA is expected to praise the country for its current co-operation.
Libya made a surprise announcement in December that it was scrapping its weapons programmes in a bid to end its international isolation.
At the weekend, the country sent all its known remaining nuclear weapons-related equipment to the US as part of a disarmament deal.
An IAEA spokeswoman said Libya, as well as Niger, would sign up on Wednesday to the agency's additional protocol, which allows tougher inspections of their atomic sites.
Another issue being raised at the Vienna talks is growing concern over the black market in nuclear material and equipment.
This follows recent revelations that a top Pakistani scientist sold nuclear secrets to Iran, North Korea and Libya.
Mr ElBaradei said he would be putting forward steps to tighten control over nuclear activities.