Iran has resumed building centrifuges used for enriching uranium, Foreign Minister Kamal Kharrazi has announced.
Iran has been accused of keeping some of its nuclear activities secret
He said the move was in response to a failure by European countries to honour a pledge to help close an international dispute over Iran's nuclear activities.
However, Mr Kharrazi said Iran had not resumed actually enriching uranium.
UN experts have been investigating suspicions that Iran is trying to build nuclear arms. Tehran insists its nuclear programme is purely peaceful.
The Iranian foreign minister told journalists in Tehran on Saturday: "We have now started manufacturing centrifuge parts".
Centrifuges are used to enrich uranium.
Iran says its nuclear programme is designed to produce energy - but highly enriched uranium can also be used for nuclear weapons.
Mr Kharrazi made clear that Iran remained committed to the suspension of enrichment activities agreed last October with Britain, France and Germany.
That deal was designed to build trust with the UN nuclear watchdog, the International Atomic Energy Agency (IAEA).
In February Iran agreed to widen the scope of the agreement and suspend the building of centrifuges.
In return, the three European powers - the so called Big Three - agreed to help Iran resolve its problems with the IAEA.
However in June the agency passed a resolution that was critical of Iran.
It Tehran's co-operation had "not been as full, timely and proactive as it should have been" and called for more transparency.
The agency described the discovery of bomb-grade uranium at Iranian facilities and an Iranian attempt to buy centrifuges as causes for concern.
Iran says the traces of high-grade uranium came from contaminated equipment imported from abroad.
IAEA chief ElBaradei has called for Iran to "come clean"
It insists it has met the IAEA's demands.
"Since the Europeans failed to meet their commitments, we can manufacture centrifuge parts," Mr Kharrazi said.
The US has accused Iran of trying secretly to build nuclear weapons.
On Thursday US Secretary of State Colin Powell said it was "more and more likely" that Iran's nuclear programme would be referred to the UN Security Council, which could impose sanctions.
Talks between the Big Three and Iran are underway in Paris to try to resolve the nuclear dispute.