Iran says it has no intention of abandoning plans to resume sensitive nuclear work, as the European Union warns that such a move could spark an international crisis.

The foreign ministers of Britain, France and Germany Tuesday signed a joint letter warning Iran against resuming activity at its Isfahan nuclear facility.The letter threatens other unspecified courses of action if Tehran does not adhere to past commitments with the European Union.

The United States says it agrees with European officials that Tehran must maintain its suspension on uranium enrichment activities.

A State Department spokesman Tom Casey says if Iran were to break its commitments with the European Union, U.S. officials would refer the situation to the United Nations Security Council and the U.N. International Atomic Energy Agency.

Iranian officials say their plan to resume nuclear work is not reversible.