VIENNA (Reuters) - Big powers voiced deep concerns on Wednesday about Iran's offer to send some of its nuclear material abroad, hours before an expected U.N. Security Council vote to slap new sanctions on the Islamic Republic.

The litany of objections from Russia, France and the United States dealt a major setback for the plan brokered by Turkey and Brazil last month even though Western diplomats insisted the response did not amount to an outright rejection.

Iran has threatened to shun any further diplomatic outreach if new, extended sanctions are imposed.

"I don't know if it (plan) is dead but it is clearly increasingly irrelevant," said Mark Hibbs, nuclear expert at the Carnegie Endowment for International Peace.


Turkey and Brazil resurrected parts of an eight-month-old U.N.-brokered plan for Iran to send 1.2 tonnes of its low enriched uranium (LEU) to Turkey in return for reactor fuel, in the hope that this would remove the need for sanctions.

The original plan, agreed in October, had been seen as a way to ease nuclear tensions by removing an amount of LEU that could have been used for an atomic bomb, if enriched to high levels.

"Only when faced with the prospect of sanctions did Iran express any semblance of accepting a proposal...made eight months ago," U.S. envoy Glyn Davies told a meeting of the International Atomic Energy Agency's 35-nation governing body.

"Iran must understand that failure to address the concerns of the international community will lead to increased pressure and isolation," he added.


Russia, France and the United States outlined nine common points of concern in writing to IAEA chief Yukiya Amano, who passed the message on to Tehran.

The main worries listed include the doubling of Iran's low-enriched uranium stockpile since the original draft deal was struck, and its decision to escalate enrichment to a higher threshold in February, diplomats said.

The launch of enrichment from 5 percent up to 20 percent fissile purity sparked particular concern because it took the material closer to the level of refinement suitable for an atom bomb. Iran says its nuclear work is for peaceful energy only.

The three powers also noted that the plan did not ensure Tehran would address broader concerns about the nature of its atomic programme, that it gives no set date for the material to be removed, and is not technically "realistic" in the set time frame.


"This is not a rejection of the idea, it is raising concerns about the plan," a senior Western diplomat said. "If Iran comes back in a serious fashion, talks are possible."

Iran, the world's fifth-largest crude oil exporter, said only that it had received the letters. France urged Tehran to give "precise answers" addressing the concerns.

(Additional reporting by John Irish in Paris and Parisa Hafezi in Tehran; Editing by Mark Heinrich)