"If we are not going to receive a clear response, a clear message from them, we are going to have no choice but to pursue additional measures," spokesman Gonzalo Gallegos said.

Negotiators from United States, Britain, France, Germany, Russia and China "have scheduled a conference call for tomorrow," the spokesman said.

"So first we will see what happens with the rest of the day that is left in Europe, and then they will have their call tomorrow. We will see where we go from there."

Gallegos also said a letter from Iran had been forwarded "recently" to Washington from the office EU foreign policy chief Javier Solana.

Without disclosing the letter's content, Gallegos said the US government "will evaluate that response. And we'll have more to say about it tomorrow after the call."

An EU source in Brussels told AFP that Iran had promised in the letter to respond to an offer from six world powers "but that clarification is needed on certain points" of the proposal.

The six world powers had offered Iran negotiations on a package of technological incentives if it suspends the sensitive process of uranium enrichment, which the West fears could be used to make nuclear weapons.

Britain, France and the United States had warned that Iran would face a fourth set of UN Security Council sanctions if it failed to produce a response to the package by Tuesday.

Adding to the confusion, a source close to Solana said that "for the time being we have not received anything" from Tehran, and emphasized there was often a time lag between Iranian announcements and the actual delivery.

The State Department said it also had no word about the status of the message.

"We checked a while ago with Solana's people and they have not received anything from them," Gallegos said.

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