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.