European Union foreign ministers have warned Iran they will refer it to the UN Security Council unless it returns to talks on its nuclear programme.
Iran says its nuclear programme is for purely peaceful purposes
EU external relations commissioner Benita Ferrero-Waldner said Iran should not underestimate European resolve.
The UN's nuclear watchdog is due to report on Saturday on Iran's nuclear activities, following its decision to resume nuclear enrichment last month.
Iran denies US claims it seeks nuclear arms, saying its purposes are peaceful.
Referral to the UN Security Council could eventually lead to sanctions against Iran.
Negotiators from France, Britain and Germany suspended talks with Tehran after it resumed uranium conversion - but have said the door remains open for a return to negotiations.
Ms Ferrero-Waldner said this should not be seen as a sign of weakness.
"The Iranians should not make the mistake of underestimating the strength of us in Europe," she told reporters.
"Nobody wants to go to the Security Council but it might become unavoidable if they don't co-operate."
However, speaking after the meeting of EU foreign ministers in Wales, UK Foreign Secretary Jack Straw said they would take matters "one step at a time".
They would wait for Saturday's International Atomic Energy Agency (IAEA) report before deciding whether to go to the Security Council, he said.
French Foreign Minister Philippe Douste-Blazy added: "We are not closing the door. Iran can come back within the 2004 negotiations framework."
By resuming its nuclear work, Tehran turned down an offer of economic incentives by France, Britain and Germany.
Iran's new chief nuclear negotiator, Ali Larijani, said last week that Tehran would come up with new proposals for talks with the EU within a month.
The US last month criticised an independent investigation which found no evidence that Iran was working on a secret nuclear weapons programme.
The report said traces of bomb-grade uranium in Iran's nuclear facilities came from contaminated Pakistani equipment, not Iranian activities - backing up Tehran's position.