"The Foreign Ministers of France, Germany and the United Kingdom and the High Representative of the European Union are extremely concerned by the latest announcements that Iran is restarting uranium enrichment activities at the Fordow facility, as confirmed by the International Atomic Energy Agency (IAEA) in its November 11 report," they said in a joint statement.

Paris, Berlin, London and Brussels said that Iran's action was "inconsistent" with the 2015 Joint Comprehensive Plan of Action, or JCPoA, under which Tehran had agreed to curb some of its nuclear activities -- notably uranium enrichment -- in return for a lifting of economic sanctions.

But the Islamic republic has said it no longer feels bound by the JCPoA after the US unilaterally pulled out of the accord last year.

Tehran's latest decision "represents a regrettable acceleration of Iran's disengagement from commitments under the JCPoA, including exceeding the maximum allowed low enriched uranium stockpile and the maximum allowed enrichment limits," the joint statement said.

The IAEA, in its latest report on Iran on Monday, said it has detected uranium particles at an undeclared site in Iran.

The report also confirmed that Iran has ramped up uranium enrichment, with its stockpile now reaching the equivalent of 551 kilogrammes, as opposed to the 300-kilogramme limit laid down in the JCPoA.

"We underline the importance of the full and effective implementation of the JCPoA by all sides and confirm our determination to continue all efforts to preserve the agreement, which is in the interests of all," the joint statement from France, Germany, Britain and the EU said.

"Iran must return to full implementation of its commitments under the JCPoA without delay."