Iran confirmed it will resume research on nuclear fuel today in a move that could see it referred to the UN Security Council for possible sanctions.
European Union and US officials say if Iran goes ahead with the move, diplomatic efforts to settle the issue would be endangered lead to the matter being sent to the UN Security Council.
International Atomic Energy Agency (IAEA) chief Mohamed ElBaradei said the world was "running out of patience with Iran".
Austrian foreign minister Ursula Plassnik, whose country holds the EU presidency, said Iran's decision was "the wrong step in the wrong direction and is a cause of very serious concern".
But Iran insists its nuclear ambitions are entirely peaceful and says it has the right to enrich uranium on its own soil.
The EU and the United States back a plan put forward by Moscow for Iran to enrich uranium in Russia, which would ensure the uranium was only enriched to levels where it could be used to generate electricity and not to make nuclear weapons.
"We will remove the seals and we have announced that we are ready to start research from tomorrow," an Iranian Foreign Ministry spokesman yesterday.
Mr Asefi said Iran would restart work when the IAEA was ready to supervise the removal of seals it put in place two years ago to freeze activities while talks went ahead.
But the IAEA said two letters sent by Iran to explain its move still left key questions unresolved and the Vienna-based agency said it had asked for more information. If Iran complies with the IAEA requests, the restart of nuclear work could be delayed.