Earlier the United States said that world powers would discuss their next move on Wednesday and indicated that Tehran faced a fourth set of UN Security Council sanctions if it failed to give a positive response.

Iran's demand for more information came in the form of a letter to EU foreign policy chief Javier Solana, which was also circulated late Tuesday to the five UN Security Council permanent members and Germany who are seeking to persuade Tehran to end its uranium enrichment programme.

The Iranian authorities "say there will be a response but that clarification is needed on certain points of the offer," the European source told AFP.

"They've hit the ball back. We must analyse whether it's just another way to gain time or if it's serious," she added.

A source with Iran's Supreme National Security Council told AFP that a message had been handed over to Solana, who has been leading negotiations on behalf of the six -- Britain, China, France, Germany, Russia and the United States.

But the source, who asked not to be identified, confirmed that the message did not contain the final response that world powers had been awaiting.

"The message delivered today is not Iran's response to the six countries," the source said.

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.

Tehran has steadfastly refused to suspend its uranium enrichment activities, which it says are only aimed at producing fuel for nuclear power for a growing population.

The United States said Iran faced "additional measures" if it did not respond clearly, indicating it risked further UN Security Council sanctions.

"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," US State Department spokesman Gonzalo Gallegos said.

Negotiators from the six powers have scheduled a conference call for Wednesday, the spokesman said.

Along with the threat of further sanctions, Washington has warned that the option of military action against Iran remains open if Tehran sticks to its defiant line.

Amid the continued tensions, Iran said on Monday it had successfully test-fired an anti-ship missile with a range of 300 kilometres (180 miles) that would allow it to close the Strait of Hormuz between Iran and Oman.

"Given the equipment our armed forces have, an indefinite blockade of the Strait of Hormuz would be very easy," said the commander of the elite Revolutionary Guards, General Mohammad Ali Jafari.

But Pentagon press secretary Geoff Morrell said that any move by Iran to close the Strait of Hormuz would be "self-defeating" because its economy is so heavily dependent on oil revenues.

"They have a very weak economy at this point which depends almost entirely on their oil revenue," he noted.

Libyan leader Moamer Kadhafi, who has sought closer relations with the West after abandoning his own nuclear ambitions in 2003, described Iran as "arrogant" and predicted it would not withstand any US attack.

OPEC member Iran is the world's fourth-biggest crude oil producer and traders fear supply disruption from the Islamic republic if tension is further heightened.

However world oil prices -- which have been driven to record levels by worries over a possible military strike on Iran -- slumped beneath 119 dollars a barrel on US economic news.

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