Iran has formally rejected key parts of a deal to send abroad for processing most of its material that could be used to make nuclear arms, diplomats said, a response the United States rejected as inadequate.
Diplomats said Iran's position, given in writing to the International Atomic Energy Agency, echoed two months of verbal calls for amendments to the deal that Western powers dismissed as non-starters but said did not amount to a final response.
Under the deal, Iran would transfer stocks of low-enriched uranium abroad and in return receive fuel for a medical research reactor. The deal aimed to minimize the risk of Iran refining the material to a grade suitable for a weapon.
In Washington, State Department spokesman P.J. Crowley said Iran's response was inadequate.
"I am not sure that they have delivered a formal response but it is clearly an inadequate response," he told reporters. "I am not sure that whatever they have done, perhaps today, is any different than what they have done previously."
Iran's failure to meet an effective US deadline of December 31 to accept the plan devised in October by then-IAEA chief Mohamed ElBaradei has prompted six world powers to start considering possible tougher sanctions against Tehran.
"This written position is a non-event because it's nothing new, it just makes official what the Iranians have been saying (through the media)," said a Western diplomat, who like others asked for anonymity due to political sensitivities.
Another Vienna-based diplomat said Iran had conveyed a written answer to the IAEA and the United States, France and Russia, the other parties to the draft deal, earlier this month after three months of delay.
Officials at the UN nuclear watchdog agency in Vienna had no immediate comment. Iran's IAEA envoy could not be reached.
The fuel plan was meant to allay suspicions Tehran wants to develop atomic bombs, rather than generate electricity, from uranium enrichment by having Iran ship around 70 percent of its low-enriched uranium stockpile abroad for further refinement and conversion into fuel to keep a medical reactor running.