The State Department said the only way for Iran to address concerns over its nuclear program was to agree to a permanent halt in uranium enrichment.

"Unfortunately, we've not seen that kind of attitude from Iran, and we may have to consider what other action to take if Iran does not fulfill its obligations," department spokesman Richard Boucher told reporters.

He did not elaborate but among options available is hauling Iran to the United Nations Security Council.

Britain, France and Germany are trying to convince Iran to halt its work on the nuclear fuel cycle -- including the sensitive process of enriching uranium -- in return for a package of incentives covering trade, security and technology.

The EU's "big three" see a halt in enrichment as the only way Iran can provide the objective guarantees needed to convince the many sceptics that it is not seeking to use an atomic energy drive as a means of also developing the capability to develop a bomb.

But Iran asserted Tuesday that it had gone far enough in providing "objective guarantees" that its nuclear program was purely peaceful and warned that a refusal by the European Union to accept this would bring negotiations to a dead-end.

The United States maintains that Iran is trying to covertly develop nuclear weapons, while Tehran insists that its programme is purely meant to meet civilian energy needs.

Iran agreed in November to suspend enrichment as a "confidence-building measure", but insists the freeze will only be temporary.

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