The UN's nuclear chief has joined Washington and the EU in urging Iran not to resume production of centrifuge parts used in uranium enrichment.
Iran opened up to inspections after international pressure
"I hope Iran will go back to full suspension," International Atomic Energy Agency head Mohamed El-Baradei said en route to Moscow.
On Tuesday Iran told the IAEA in a letter that it intends to resume centrifuge activities.
Iran is permitted to make centrifuges for peaceful nuclear energy.
But centrifuges can also be used to purify uranium to make it usable as fuel for weapons.
Tehran had promised Germany, France and Britain in February it would discontinue making the centrifuges.
"It does not involve the enrichment of material, nonetheless I think it would be good for Iran to go back [to a suspension]," Mr El-Baradei said.
The US and European Union on Saturday called on Iran to "rethink
The BBC's Bethany Bell in Vienna, where the IAEA is based, says Tehran's move is being seen in western circles as a setback, amid suspicions that it is trying to develop a nuclear weapons programme.
Last week, Tehran reacted angrily after the IAEA passed a resolution which "deplored" the fact that "Iran's co-operation has not been as full, timely and proactive as it should have been".
The IAEA expressed serious concern that important information about Iran's P2 centrifuges, which can be used to produce bomb-grade uranium, had been incomplete and unclear.
Senior figures in the Iranian government say Iran is no longer bound by its commitments to the three EU nations, because, as they see it, the countries broke a pledge to help wrap up the IAEA investigation - an inquiry now set to continue for a few months at least.
Tehran rejects US allegations that its nuclear programme is being used to make weapons and says it is solely for generating electricity.