Mohamed ElBaradei asked Iran to clarify its response to the U.S.-backed and U.N.-brokered proposal. In mixed signals, Iran's foreign minister said that option had not been rejected, while a senior diplomat suggested the opposite.

U.S. Secretary of State Hillary Rodham Clinton said the proposal would not be changed.

Iranian Foreign Minister Manouchehr Mottaki, speaking to reporters in Kuala Lumpur, Malaysia, said no when asked if Iran had rejected the plan.

He said Iran was mulling the plan as one of three options in procuring fuel for a research reactor. The two others would be to buy the fuel from abroad or enrich the uranium domestically.

In contrast, Ali Asghar Soltanieh, Iran's chief envoy to the IAEA, said Iran would prefer to purchase enriched uranium abroad for its reactor.

"We want to buy the fuel from any supplier," he said in an interview, fending off repeated questions on whether this meant the rejection of the export plan.

Soltanieh's comments were the most concrete statement yet by a government official of what Iran wants.

Also, Mohammad Khazaee, Iran's ambassador to the United Nations, blasted ElBaradei for failing to cite Israel, which is widely believed to possess nuclear weapons, for failing to comply with the Nuclear Nonproliferation Treaty.

The proposal would have Tehran export 70 percent of its enriched uranium - enough to make one bomb - and then have it reprocessed and returned as fuel suitable only for its research reactor and not for weapons.

Addressing the U.N. General Assembly, ElBaradei said "a number of questions and allegations relevant to the nature" of Iran's nuclear program remained, and he called for confidence-building measures on all sides.

"I therefore urge Iran to be as forthcoming as possible in responding soon to my recent proposal, based on the initiative of the U.S., Russia, and France, which aimed to engage in a series of measures that could build confidence and trust," ElBaradei said in his final address before stepping down at the end of the month. He has been chief of the International Atomic Energy Agency for 12 years.

"This is a unique and fleeting opportunity to reverse course from confrontation to cooperation and should, therefore, not be missed," he said.

The United States and other powers fear Iran is enriching uranium for use in nuclear weapons. Iran insists its program is for research and energy production.

Iran's mixed messages appeared geared toward pushing the plan's backers into further talks, something those countries oppose as a delaying tactic.

"We continue to press the Iranians to accept fully the proposal that has been made, which they accepted in principle," Clinton told reporters in Morocco after consulting with senior government officials from several Persian Gulf nations, plus Egypt, Morocco, and Jordan.

"Acceptance fully of this proposal . . . would be a good indication that Iran does not wish to be isolated and does wish to cooperate with the international community," she said, adding that Iran should accept it as it stood, "because we are not altering it."

Some experts said Iran might be in no hurry to cut a deal. "Iran believes time is on their side for now," said Mustafa Alani, a regional analyst at the Gulf Research Center in Dubai.

The row allows Iran's President Mahmoud Ahmadinejad - embattled after allegations of fraud tainted his June reelection - to reclaim the high ground as defender of Iran's national dignity through its strides in nuclear technology.