Iran has clearly violated its United Nations nuclear safeguards obligations, the United States has said.
Tehran denies it has a nuclear weapons programme
"The United States believes that the facts... would fully justify an immediate finding of non-compliance by Iran," US ambassador Kenneth Brill said in written remarks to UN nuclear watchdog's governing board.
But Mr Brill, who serves as ambassador to the UN in Vienna, said Washington had consented to other board member states' desire "to give Iran a last chance to stop its evasions".
The International Atomic Energy Agency (IAEA) is holding a week-long series of meetings focusing on concerns about Iran's nuclear programme.
IAEA Director General Mohammed ElBaradei said the board members called on Iran to show full transparency and "accelerated co-operation".
Mr ElBaradei said he hoped Tehran would Iran provide all details of its nuclear activities in the coming weeks.
The United States accuses Iran of trying to develop nuclear weapons and wants the UN agency to condemn it for violating the Nuclear Non-Proliferation Treaty.
The closed-door session of the IAEA's governing board is examining a report by its inspectors, which says traces of weapons-grade uranium were found at an Iranian nuclear plant.
Iran denies its uranium enrichment activities are part of an illegal weapons programme.
It says it is seeking to produce only low-grade uranium fuel to meet its energy needs.
Mr ElBaradei urged Tehran "in the next few weeks ... [to] clarify all the important issues that are outstanding, particularly in regard to the enrichment programme".
He said the UN nuclear agency "would like to resolve this issue as early as possible" but would not be setting a deadline.
Mr ElBaradei hoped Tehran would sign up to an additional protocol that would allow intrusive snap inspections of its nuclear facilities.
The IAEA chief said he had been encouraged by statements by the Iranian delegate that his country was already working as if it had already signed the protocol.
If Iran is found in non-compliance of its nuclear obligations, as the Americans allege, the matter can be taken up by the UN Security Council.
The widely leaked report says that, in August, Iran belatedly admitted that it had carried out uranium conversion experiments in the early 1990s which should have been declared.
The report insists that Tehran needs urgently to clarify certain key questions, not least about the traces of weapons-grade uranium found by agency inspectors at the enrichment plant at Natanz.
But it stops well short of finding Iran in non-compliance with its obligations under the non-proliferation treaty.
For its part, Iran insists its nuclear activities are peaceful, devoted solely to generating electricity, and has warned that too much pressure from the US and its allies could be counterproductive, aggravating tensions rather than easing concerns.