Britain said the lack of a positive response from Tehran by the end of the day would leave the powers with "no choice" but to ask the UN Security Council to take further punitive measures.

The new deadline was set after Iran ignored a previous ultimatum to formally respond by the weekend to the incentives package being offered by the United States, Britain, France, Germany, Russia and China.

It also came as Iran said it had successfully test-fired an anti-ship missile with a range of 300 kilometres (180 miles) that would allow it to close a critical Persian Gulf waterway to oil shipments.

"No enemy vessels would be able to escape it" the commander of the elite Revolutionary Guards, General Mohammad Ali Jafari, said on state television.

"Given the equipment our armed forces have, an indefinite blockade of the Strait of Hormuz would be very easy," Jafari said, referring to the strategic waterway through which much of the world's oil supplies pass on their way out of the Gulf.

Iran has in recent months frequently boasted of developing new weapons and military hardware but the claims have often met with skepticism from Western defense analysts.

On Monday, EU foreign policy chief Javier Solana held what a spokesman described as "inconclusive" talks with Iran's top nuclear negotiator, Saeed Jalili.

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

The United States and its allies fear the program is a cover for developing nuclear weapons.

The United Nations has imposed three sets of sanctions against Iran over the dispute and is mulling a fourth round of measures.

Iran's foreign ministry spokesman Hassan Ghashghavi dismissed the idea of a deadline as "media speculation" and insisted that negotiations were an "ongoing process".

Iranian state-run television said that in the Solana-Jalili telephone conversation, "both sides agreed to continue talks," and Solana's spokesman said that further contacts between the EU and Iran "are not ruled out in the coming days."

"They also emphasized that preserving this path (talks) needs a positive and constructive atmosphere," the Iran television report said without elaborating.

But France echoed Britain in suggesting that the time had come for stronger action.

"If we don't get an encouraging response from the Iranians, we will have to show firmness, resort to sanctions as in the past," France's deputy UN ambassador Jean-Pierre Lacroix told AFP in New York.

The package proposed to Tehran offers technological incentives if it suspends uranium enrichment.

US State Department spokesman Gonzago Gallegos said Jalili had told Solana that Tehran would provide a written response on Tuesday.

Senior diplomats from the United States, Britain, France, Germany, Russia and China discussed the latest developments by telephone Monday.

"We agreed that in the absence of a positive response, we have no choice but to pursue further measures," Gallegos said.

"We are disappointed that we have not yet received a response from Iran as requested in Geneva on July 19," he said.

Solana presented an offer of economic and trade incentives in mid-June, while Iran has put forward its own proposal, an all-embracing package of suggestions to resolve the problems of the world, including the nuclear issue.

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