"When we ask questions in Iran, we ask them because we want to reconstruct the 'history'. What did Iran procure? Who was involved? What was a certain experiment for? When and where did it take place?," ElBaradei said.

The comments were made last week at the end of a closed-door session of the International Atomic Energy Agency's 35-nation board of governors, in which they shelved indefinitely appeals for technical aid for an Iranian nuclear reactor.

Tehran insists the reactor is for peaceful purposes only, but the United States argues it could produce plutonium for nuclear weapons.

ElBaradei said the aid was being turned down due to a lack of confidence in Iran's nuclear program, at a time when the UN Security Council is considering imposing sanctions on Tehran for defying its call to suspend uranium enrichment.

Iran agreed last week to hand over records of its uranium enrichment work and to allow environmental sampling at a crucial site in a boost to UN efforts to determine whether Tehran seeks nuclear weapons, but diplomats and analysts said more cooperation is needed.

The IAEA has other outstanding issues it wishes to clear up but Iranian ambassador Ali Asghar Soltanieh made clear that there would be no more such steps unless the UN Security Council stops threatening Iran with sanctions.

ElBaradei had told his board in a regular report that limited cooperation by Iran had blocked the IAEA from making "further progress" on clearing up questions about Tehran's nuclear program, including the scope of its enrichment work.

Diplomats described ElBaradei's impromptu speech that closed the board meeting on Thursday as indicative of his frustration that, after more than three years of investigation, the IAEA is still unable to conclude the true nature of the Iranian program.

The IAEA does know "that Iran has knowledge over the entire spectrum of the fuel cycle," ElBaradei said.

While this is legitimate under the nuclear Non-Proliferation Treaty, "it is something that should have been declared to us years back."

Nuclear reactor fuel, such as enriched uranium, can also provide material for atom bombs.

The IAEA investigation began in February 2003 after it was revealed that Iran had been hiding nuclear work for almost two decades.

"ElBaradei has staked all his reputation and career on working with the Iranians," a Western diplomat said of the IAEA chief's insistence on peaceful diplomacy to defuse the nuclear crisis.

ElBaradei's comments showed that he saw Iran's lack of cooperation with IAEA inspectors "as a direct challenge to everything he has been trying to do," the diplomat added.

While the IAEA has managed to verify that all declared nuclear material and facilities in Iran are under safeguards, ElBaradei said it lacked reassurance "that there is nothing in Iran that has not been declared to us."

A unique element in Iran's case was that the IAEA investigation was unable to start its work on a clean slate, he said.

"We started from a situation where we came to realize that there had been activities for 20 years which we did not know about."

"Obviously that creates a different situation and means that Iran must take the initiative to explain what happened," ElBaradei said.

He said he was "basically telling Tehran, 'if you want to fully restore the confidence of the international community you need to go out of your way to clarify the situation for us."

That means allowing the IAEA to interview key officials as well as "getting records, having evidence of what happened," ElBaradei said.

US ambassador to the IAEA Gregory Schulte told a university seminar in Vienna Monday that the US estimate is that Iran could have a nuclear bomb as early as the beginning of the next decade.

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