Iran has said that it will not stop uranium reprocessing work, rejecting a European threat that Tehran had less than two weeks to freeze uranium conversion or face referral to the UN Security Council for possible sanctions.
Iran resumed uranium reprocessing activities at its facility in Isfahan last month after it rejected a European package of proposals.
That package had called on Iran to permanently stop its uranium enrichment programme in return for a supply of nuclear fuel and economic incentives.
Iran says it will not give up uranium enrichment, a right granted to it under the Nuclear Non-Proliferation Treaty.
A report by Mohamed ElBaradei, head of the International Atomic Energy Agency, said on Friday that Tehran had recently produced about seven tonnes of the gas it needs to enrich uranium - a possible pathway to a nuclear weapon.
"The issue of Isfahan is a thing of the past. We won't get back it," Foreign Ministry spokesman Hamid Reza Asefi said on Sunday.
The US has accused Iran of using its civilian nuclear programme as a cover to secretly produce nuclear weapons.
Iran has rejected the charges, saying its nuclear programme is geared towards generating electricity, not making a bomb.