"That's precisely what we're looking for. We're looking for an acknowledgement that they had a nuclear weapons programme," the US permanent representative to the UN nuclear watchdog, Gregory Schulte, told reporters.

"And we're looking for them to cooperate fully with the agency in understanding their current activities," Schulte said.

The US envoy was responding to questions about how he thought the long-running nuclear deadlock between Tehran and the rest of the world could be resolved.

"Iran should admit its past weapons-related activities rather than trying to cover them up. Iran must come clean," Schulte said.

Tehran "owed" the Vienna-based International Atomic Energy Agency (IAEA) "a full disclosure of its past activities, the 'confession' repeatedly requested by (IAEA chief Mohamed) ElBaradei," he said.

Earlier this month, the National Intelligence Estimate -- the consensus view of all 16 US spy agencies -- declared with "high confidence" that Iran had had a nuclear weapons programme, but halted it in 2003.

The report was a diplomatic bombshell, contradicting forceful US assertions that Iran's nuclear programme was a gathering threat that raised the prospect of "World War III."

Iran hailed the report as a "victory", saying it proved wrong US claims that it is currently seeking to develop nuclear weapons.

But for the United States and many of its western allies, the report only underlined the need for urgent action, since Iran could resume the weapons programme at any time, without the knowledge of the IAEA.

"We have new evidence and are highly confident that Iran had a nuclear weapons programme, only four years ago. And Iran's leaders could choose to restart that programme, just as they have restarted uranium enrichment," Schulte said.

"There is no certainty that the IAEA would know, particularly as ElBaradei has twice warned us that the IAEA's knowledge of Iran's current activities is diminishing," he said. "This is a matter of grave concern."

Schulte insisted that Iran suspend all uranium enrichment-related activities in order to persuade the world of the peaceful nature of its contested atomic drive.

"Iran's continued pursuit of an enrichment capability violates (UN) Security Council resolutions and casts doubts on its leaders' ultimate intent," he said.

"Iran is already a source of danger in the Middle East. That danger only increases as Iran's leaders shorten the timeline to produce nuclear weapons."

Iran refuses outright to suspend uranium enrichment, insisting on its inalienable right to develop nuclear energy to meet the needs of a growing population.

But western countries, and the United States in particular, insist that suspension is a pre-condition if there is any chance of a negotiated settlement being reached.

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