Germany and the UK have voiced concern at Iran's decision to resume production of centrifuge parts used in uranium enrichment.
Iran opened up to inspections after international pressure
The two countries said they were working with France on a response.
A UK Foreign Office spokesman told the BBC they were disappointed and did not understand Iran's move.
Last week, the UN's atomic agency sharply rebuked the Tehran government for failing to co-operate fully with an inquiry into its nuclear activities.
Iran informed the three European nations and the UN body, the International Atomic Energy Agency, in writing that it would resume making centrifuge parts and the assembly and testing of centrifuges.
In the letter, a copy of which was obtained by the BBC, Iran said it would begin these activities on 29 June.
Under international treaties, Iran is allowed to make centrifuges for peaceful nuclear energy.
But 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.
Iran insists its nuclear activities are peaceful [Photo: Digitalglobe]
For US Undersecretary of State for Arms Control John Bolton, the letter was proof of Iran's intent to reprocess uranium as part of a covert weapons programme.
"This is an act of defiance of the IAEA board of governors; it is a thumb in the eye of the international community," he said.
The European governments were more circumspect, speaking of their disappointment and incomprehension at the move.
"The foreign ministry in Berlin regrets the announcement made by the Iranian government, " a German spokesman said.
A source in Paris told the French news agency, AFP, they were working with the British and Germans towards "a common and coordinated position on the matter".
Last year, Iran reached a deal with the three countries to suspend its uranium enrichment activities - a move seen as a confidence-building measure.
But 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.