Iran is expected to respond today to a crucial international offer regarding its nuclear programme, amid concerns among world powers that it will not accept a deal to transfer about two- thirds of its current stock of low-enriched uranium to France and Russia.

Iranian media again signalled yesterday that Tehran would be seeking changes to a draft plan drawn up by the International Atomic Energy Agency, a move that could force the deal to unravel if its demands were seen as excessive by world powers.

In an interview with the Financial Times, David Miliband, Britain's foreign secretary, said the offer, if it were accepted by Iran, would not resolve many concerns over its nuclear programme, such as allegations that it has sought to militarise the programme in the past.

"But it is an important test, and it's an important benefit for Iran because the fabricated enrichment would return to Iran for medical use. So it's a potential to show a win-win."

"Acceptance of the medical isotopes deal is not a full answer to the concerns. But it's an important component of bringing discipline and I suppose respect, and space for this issue to be addressed properly."

These developments came as a bill that would give the Obama administration power to impose sanctions on a range of companies involved in providing Iran with petrol cleared a key congressional committee.