Iran's resumption of research work on nuclear fuel violates an agreement with European Union countries to hold back from sensitive nuclear activities, the 25-country bloc said on Tuesday.
"It is considered a violation of the Paris agreement. These activities are clearly related to enrichment," Cristina Gallach, spokeswoman for EU foreign policy chief Javier Solana, said by telephone.
"This is very much a step in the wrong direction. We are extremely concerned and consultations are taking place (within the EU) to coordinate a response."
Tehran announced on state television earlier that its nuclear research centres had restarted their activities.
Asked whether the move increased the likelihood of a referral to the UN Security Council for possible sanctions, Gallach replied: "It is part of the consultation to consider further steps."
Solana and ministers from Britain, France and Germany -- the so-called "EU3" states leading negotiations with Iran aimed at persuading it to shelve its nuclear programme -- are due to meet in Berlin on Thursday to consider their next step.
The United States and others in the West suspect Tehran of seeking atomic weapons, while Iran insists it needs nuclear plants to satisfy its energy needs.