Iran said it would break UN seals on a nuclear plant today and resume work there, defying EU warnings such a step could crush hopes of a negotiated solution.
Two years of hard bargaining over a nuclear programme that Tehran had kept secret for 18 years appeared to be heading towards a crisis that could see Iran's case sent to the United Nations Security Council for possible sanctions.
The EU "Big Three" of Britain, France and Germany have been trying to mediate between the United States, which insists Iran is trying to produce nuclear weapons, and the Islamic Republic which says it has a right to develop peaceful atomic technology.
"Iran is to remove the seals today," the spokesman of the Supreme National Security Council Ali Aghamohammadi told reporters.
The conversion plant near the central city of Isfahan takes processed uranium ore, mined in Iran's central desert, and turns it into uranium hexafluoride gas. This gas can be pumped into centrifuges that spin at supersonic speed to enrich uranium.
Enriched uranium is used in nuclear power plants, but if highly enriched can be used in atomic weaponry.
The European Commission today urged Iran not to go ahead with its plan. A spokesman for the European Union executive told a press briefing: "The Commission very much hopes for a negotiated solution. We would also hope that no steps would be taken over the coming days to endanger such a negotiated solution."
Britain, Germany and France are due to put to Tehran a package of proposals for nuclear, economic and political incentives provided Iran renounces nuclear enrichment-related activities.
The EU has said if Iran went ahead and resumed work, then as a first step it would urgently consult the board of the UN's nuclear watchdog, the International Atomic Energy Agency.
The UN's nuclear watchdog can recommend referring Iran to the Security Council, which could decide whether to impose sanctions.