Under the plan thrashed out in talks with France, Russia and the United States, Iran was to have shipped out most of its stocks of low-enriched uranium in return for fuel for a research reactor in Tehran.

The proposals were designed to assuage fears that Iran could otherwise divert some of the stocks and enrich them further to the much higher levels of purity required to make an atomic bomb.

But officials, who strongly deny any such intention, had expressed mounting concern that Iran's arch-foe Washington might welch on the deal and Tehran might ship out its uranium without receiving anything in return.

"We do not want to give part of our 1,200 kilos (more than 2,640 pounds) of enriched uranium in order to receive fuel of 20 percent enrichment," said Alaeddin Borujerdi, the influential head of parliament's national security and foreign policy committee.

"This option of giving our enriched uranium gradually or in one go is over now," he told the ISNA news agency.

"We are studying how to procure fuel and (Ali Asghar) Soltanieh is negotiating to find a solution," he added, referring to Iran's envoy to the UN nuclear watchdog.

A spokesman for the International Atomic Energy Agency said on Saturday that it was "still waiting for the formal response" from Soltanieh.

But a second member of the Iranian parliamentary committee also said that the response would be negative.

"We were not against the exchange," Iran's ISNA news agency quoted Hossein Naghavi Hosseini as saying.

"But during the negotiation, they were unable to give Iran the confidence and so the response of the Islamic Republic of Iran is negative.

"They made the offer of exchange... but according to the NPT (Non-Proliferation Treaty), we have the right to buy the fuel we need."

Foreign Minister Manouchehr Mottaki had said on Friday that Iran was considering the alternative of trying to seal a deal with a foreign government to import the fuel for its Tehran reactor commercially or further enrich its own uranium.

But Russia, which would have further processed the Iranian uranium under the proposed deal with the powers, warned the Tehran authorities that they risked further UN sanctions if they took a "less constructive position."

"I do not want that all this ends up with the adopting of international sanctions because sanctions, as a rule, lead in a complex and dangerous direction," President Dmitry Medvedev said in comments released by the Kremlin on Saturday.

"But if there is no movement forward then no-one is going to exclude such a scenario," he said in the interview with German weekly Spiegel.

In its initial reply to the IAEA-drafted plan handed over on October 29, Tehran had taken issue with provisions for it to ship out 75 percent of its stocks before receiving any fuel, Iranian media reported.

A number of senior officials had argued that that stipulation was a major concession for Iran for which it was receiving little in return.

"What guarantee do we have that if we deliver our enriched uranium, we will get the fuel?" hardline cleric Ahmad Khatami said on Friday, giving vent to the misgivings of many in the regime.

In an interview with the New York Times on Thursday, the UN watchdog's director Mohamed ElBaradei spoke of the difficulties of brokering a deal amid the legacy of suspicion between Tehran and Washington, which have had no diplomatic relations since the aftermath of the 1979 Islamic revolution.

"There's total distrust on the part of Iran," ElBaradei said.

But the IAEA chief also made plain that Iranian demands for its low-enriched uranium to be shipped out at the same time as it received the nuclear fuel were not acceptable to Western powers.

A simultaneous exchange "would not defuse the crisis, and the whole idea is to defuse the crisis," he said.

On Thursday, US Secretary of State Hillary Clinton had underlined Washington's refusal to accept any changes to the IAEA draft.

"This is a pivotal moment for Iran, and we urge Iran to accept the agreement as proposed," she said.

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