Iran has material for 1-2 atom bombs--ex-IAEA aide

PARIS, Aug 26 (Reuters) - Iran has stockpiled enough low-enriched uranium for 1-2 nuclear arms but it would not make sense for it to cross the bomb-making threshold with only this amount, a former top U.N. nuclear official was quoted as saying.

In unusual public remarks about Iran's disputed nuclear programme Olli Heinonen, the former chief of U.N. nuclear inspections worldwide, told Le Monde newspaper that Iran's uranium reserve still represented a "threat."

Until he stepped down earlier this month for personal reasons, Heinonen was deputy director-general of the International Atomic Energy Agency and head of its nuclear safeguards department, which verifies that countries' nuclear programmes are not being diverted for military use.

A no-nonsense Finn, he was one of the U.N. agency's leading experts on Iran, which denies Western suspicions that its nuclear programme is aimed at making bombs despite intelligence indications to the contrary, which he investigated for years.

In the interview published on Thursday, Heinonen said the Islamic Republic now possessed three tonnes of low-enriched uranium, material which can be used to fuel nuclear power plants, or form the core of a bomb if refined much further.

"In theory, it is enough to make one or two nuclear arms. But to reach the final step, when one only has just enough material for two weapons, does not make sense," Heinonen said in the interview carried out just before he left office.

In comments translated from English to French, he suggested this was not sufficient to constitute a serious bargaining chip in any negotiations with the United States, the Islamic Republic's old adversary.

"But this constitutes a ... threat," he said, apparently referring to Iran's LEU stockpile.

Heinonen said the United States estimated that Iran would need a year to convert its low-enriched uranium to higher-grade material, adding that this was a not a "bad estimate."

Top Pentagon officials told the U.S. Congress in April that Iran could produce enough highly enriched uranium for a single nuclear weapon in as little as a year -- but would probably need three to five years to assemble, test and deploy it.

World powers hope that new U.N., U.S. and European sanctions imposed on Iran since June will persuade it to enter negotiations on its nuclear programme which the West hopes will lead to a suspension of all uranium enrichment activity.

Iran, which says its nuclear work is aimed at generating electricity so that it can export more of its gas and oil, has repeatly ruled out halting enrichment, while keeping the door open for talks.

Heinonen is probably best known for giving a closed-door presentation to diplomats on Iran in 2008 which indicated links between projects to process uranium, test explosives and modify a missile cone in a way suitable for a nuclear warhead.

His department's five-year investigation based on Western intelligence funnelled to the agency helped harden IAEA concerns that Iran might have worked to develop a nuclear-armed missile and was still doing so.

Tehran says the intelligence is forged and that its atomic work is solely for peaceful purposes.