"Iran must immediately end this constant violation of resolutions by the United Nations Security Council and the IAEA governors, rather than showing its contempt for international law," a foreign ministry spokesman said.
Earlier, Iranian nuclear chief Fereydoon Abbasi Davanihad told state media the country's efforts to enrich uranium to the 20 percent level needed for nuclear fuel would shift to a plant in Fordo and be ramped up.
Western capitals fear Iran will eventually make the leap to enrich uranium at the much higher level needed to create a nuclear weapon. They oppose any moves to improve the Islamic republic's enrichment capacity.
"The announcement is a provocation. It heightens the existing concerns of the international community over the intransigence of the Iranian regime and its constant breaches of international law," said the spokesman, Bernard Valero.
In 2009 the shock revelation of the formerly secret Fordo plant, near the Shiite holy city of Qom around 150 kilometres (94 miles) southwest of Tehran, prompted the United Nations to strengthen sanctions against Iran.
Iran warned the UN's International Atomic Energy Agency (IAEA) in February that Fordo was ready to host centrifuges – machines which enrich uranium – and that it would become operational around the middle of the year.