Iran today told the UN nuclear agency that it will start enriching uranium to higher levels, shrugging off international fears that such a move will bring it closer to being able to make nuclear warheads.
Iranian envoy Ali Asghar Soltanieh sought to dispel such concerns, saying that the uranium to be enriched to 20% would be used only to make fuel for Tehran's research reactor, which is expected to use up its present stock within a year.
Mr Soltanieh, who represents Iran at the Vienna-based International Atomic Energy Agency (IAEA), also said that IAEA inspectors would be able to fully monitor the process.
He blamed world powers for Iran's decision, asserting that it was their fault that a plan that foresaw Russian and French involvement in supplying the research reactor had failed.
"Until now, we have not received any response to our positive, logical and technical proposal," he said. "We cannot leave hospitals and patients desperately waiting for radio isotopes" being produced at the Tehran reactor and used in cancer treatment, he added.
Iranian President Mahmoud Ahmadinejad had already announced on Sunday that his country would significantly enrich at least some of the country's stockpile of uranium but today's formal confirmation of the plan was important, particularly because of the rash of conflicting signals sent in recent months by Iranian officials on the issue.
The Iranian move came just days after President Ahmadinejad appeared to move close to endorsing the original deal, which foresaw Tehran exporting the bulk of its low-enriched uranium to Russia for further enrichment and then conversion for fuel rods for the research reactor.
The plan was endorsed by the US, Russia, China, Britain, France and Germany - the six powers that originally elicited a tentative approval from Iran in landmark talks last autumn.
Since then, however, mixed messages from Tehran have infuriated the US and its European allies, who claim Iran is only stalling for time as it attempts to build a nuclear weapon.
The original plan was welcomed internationally because it would have delayed Iran's ability to make a nuclear weapon by shipping out most of its low-enriched uranium stockpile.
Although material for the fissile core of a nuclear warhead must be enriched to a level of 90% or more, getting its stockpile to the 20% mark would be a major step for the country's nuclear programme.
While enriching to 20% would take about one year, using up to 2,000 centrifuges at Tehran's underground Natanz facility, any next step - moving from 20% to 90% - would take only half a year and between 500-1,000 centrifuges.
Achieving the 20% level "would be going most of the rest of the way to weapon-grade uranium", said David Albright, whose Washington-based Institute for Science and International Security tracks suspected proliferators.
Iran has defied five UN Security Council resolutions - and three sets of UN sanctions - aimed at pressuring it to freeze enrichment, and has instead steadily expanded its programme.
It denies nuclear weapons ambitions, insisting it needs to enrich to create fuel for an envisaged nuclear reactor network.