France, Britain and Germany said they would be forced to take other “courses of action” if dialogue failed.
Iranian officials say the decision to resume uranium processing at its Isfahan plant is irreversible.
France, Britain and Germany, have so far resisted calls by the United States to take Iran to the UN, hoping that the crisis can be defused in talks.
But German Chancellor Gerhard Schroeder called Iran’s latest position “threatening”, and said the EU, US and Russia would remain united in opposing it.
Last year, Iran reached an agreement with the three EU countries on suspending uranium processing and all other nuclear fuel cycle activities, but on Monday it announced it was set to resume them.