Iran says it only wants nuclear power for peaceful purposes
Iran has told the International Atomic Energy Agency it does not accept the terms of a deal to ease concerns about its nuclear programme, diplomats say.
For months, the Iranian government has criticised the offer to ship low-enriched uranium abroad in return for fuel, but never responded formally.
But diplomats say Tehran is now suggesting an alternative involving a simultaneous exchange on its territory.
Correspondents say the proposal is very unlikely to be acceptable to the West.
The US and its allies fear Iran is attempting to develop nuclear weapons. Iran insists its nuclear programme is entirely peaceful.
The BBC's Bethany Bell in Vienna says it is not clear whether Iranian officials have responded in writing or only verbally to the IAEA about the deal that envisages Iran sending about 70% of its low-enriched uranium to Russia and France, where it would be processed into fuel.
But diplomats say they appear to have rejected one of the main conditions - that all the uranium leaves Iran well before any fuel is dispatched.
When asked about the reports on Tuesday, a US state department spokesman said Iran's proposal was inadequate.
"I am not sure that they have delivered a formal response, but it is clearly an inadequate response," P J Crowley said.
NUCLEAR FUEL CYCLE
Mined uranium ore is purified and reconstituted into solid form known as yellowcake
Yellowcake is chemically processed and converted into Uranium Hexafluoride gas
Gas is fed through centrifuges, where its isotopes separate and process is repeated until uranium is enriched
Low-level enriched uranium is used for nuclear fuel
Highly enriched uranium can be used in nuclear weapons
"I am not sure that whatever they have done, perhaps today, is any different than what they have done previously."
Neither officials at the IAEA nor Iran's representative to the organisation, Ali Asghar Soltanieh, have so far commented.
The US and its allies on the UN Security Council have been pushing for a fourth round of sanctions if Iran's rejects the deal.
But Russia and China are said to be sceptical of any new penalties.