A top US official has accused Iran of continuously changing its explanations after UN nuclear inspectors find previously undeclared activities.
The IAEA has persuaded Iran to permit tougher checks
"The Iranians change their stories to fit the facts," said Kenneth Brill, US ambassador to the UN nuclear agency.
The International Atomic Energy Agency (IAEA) would be dealing with Iran for "many years to come" Mr Brill said.
He was speaking in Vienna where the IAEA board of governors is meeting to consider how to proceed with Iran.
IAEA Director General Mohammed ElBaradei said he was "seriously concerned" about omissions in that declaration - and dismissed Iranian calls to drop the issue from the international agenda.
Iran's ambassador said Tehran had never said the dossier was complete.
Pirooz Hosseini said his country had been the victim of a "war of propaganda" and "misquoted" as saying the declaration was complete.
But according to his US counterpart, Iranian officials had said the October report would be "full, complete and represent total transparency".
"When it was proved that was not the case, then the Iranians changed their story and said we didn't mean it was going to be full and complete," said Mr Brill.
"I think its striking that the more the agency learns the more the Iranians have to change their stories," he said.
Iran had violated the nuclear Non-Proliferation Treaty (NPT) for many years, Mr ElBaradei said at the Vienna talks.
Iran wants the world to close the country's nuclear file
He singled out Iran's failure to declare that it was researching advanced centrifuge designs, known as P2, capable of producing highly enriched uranium.
This, he said, had been "a setback to Iran's stated policy of transparency".
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.