Western officials had set an informal deadline of last Saturday for Tehran to respond to an offer by the powers to refrain from imposing more U.N. sanctions on Iran if it froze expansion of its nuclear work.

Shortly before the call between Iran's chief nuclear negotiator Saeed Jalili and European Union foreign policy chief Javier Solana, an Iranian official said Iran would not discuss the freeze idea.

The West fears the programme is aimed at building bombs.

The United States said after Iran missed Saturday's deadline that the U.N. Security Council had no choice but to expand sanctions. Iran says it was never given any ultimatum.

In comments likely to stoke tension, Iran's Revolutionary Guards chief said the Islamic Republic had the ability to close the Strait of Hormuz, a vital oil shipping route, if attacked. The Guards said they had tested a new naval weapon.

Washington has not ruled out military action if diplomacy fails to end the row. Worries about a conflict flaring up have rattled oil markets in the past.

"Mr Solana will be in touch with the authorities of China, France, Germany, Russia, the United Kingdom and the United States," an EU official said after confirming the phone talks.

Iranian state radio said: "They emphasised continuing the course of the Geneva talks. The preservation of this course requires a constructive and positive environment".

The two sides would consult again in coming days. it said.

Solana and Jalili last met in Geneva.

But a senior Iranian official told Reuters before the call that "the freeze-for-freeze issue will not be discussed".

'INALIENABLE RIGHT'

The freeze idea was to get initial talks going before formal discussions on nuclear, trade and other incentives offered by the six powers.

But the powers have said those formal negotiations will only start once Tehran suspends uranium enrichment, a process that can have civilian and military uses.

Iran, the world's fourth largest oil producer, has repeatedly ruled out halting nuclear work that it says is aimed at mastering technology to generate electricity, not make bombs.

"Enrichment is Iran's inalienable right," Foreign Ministry spokesman Hassan Qashqavi told a news conference. "When it comes to our inalienable rights, we will press ahead."

Washington, leading efforts to isolate Iran, said on Sunday Iran's refusal to stop made moves on more sanctions inevitable.

"It is clear that the government of Iran has not complied with the international community's demand to stop enriching uranium and isn't even interested in trying," said Richard Grenell, spokesman for the U.S. mission to the United Nations.

"They leave the Security Council no choice but to increase the sanctions, as called for in the last resolution passed."

Russia, which opposed setting a deadline for Iran to respond, and China have been more reluctant to impose sanctions but have in the end voted for three such resolutions.

Economists say Iran has a big cash cushion from windfall oil revenues to cope with sanctions. But the penalties have hurt the economy and added costs for Iranian businesses. they said.

Western firms in particular have been steering clear of Iran.

Guards commander-in-chief Mohammad Ali Jafari said Iran would strike back if attacked and it could close the Strait of Hormuz "easily and on an unlimited basis", state media said.

It followed an announcement that the Guards had tested a new naval weapon that could destroy any vessel in a range of 300 km (190 miles).

Around 40 percent of the oil traded in the world leaves the region through the Strait of Hormuz, a choke point at the southern end of the Gulf, flanked by Iran and Oman. Washington has pledged to keep shipping routes open.