World powers denied on Monday an Iranian report that they had offered a fresh proposal for a swap on nuclear fuel, saying "the door remains open" for Tehran to accept a proposal offered in the autumn.

"The door remains open for Iran to accept the practical, fair, and responsible proposal put forth by the IAEA last fall that would enable Iran to meet its medical humanitarian needs as well as build international confidence in Iranian intentions," White House spokesman Mike Hammer said.

France also dismissed the report, saying the existing deal was the only valid offer. Russia also said the countries had simply confirmed their support for a proposal brokered by the International Atomic Energy Agency (IAEA) last year to send much of Iran's low enriched uranium abroad.

Russian Foreign Minister Sergei Lavrov said in Guatemala on Monday that the offer for enrichment abroad "remains in force and we very much hope that Iran will accept it," the Itar-Tass news agency reported.

He said there were various ways it could be implemented, but "they do not change the essence of the offer."

Earlier the semi-official ILNA news agency quoted Ali Akbar Salehi, the head of Iran's Atomic Energy organization, as saying proposals had been received after Tehran opted to step up its own uranium enrichment, a decision it announced last week.

Meanwhile, Secretary of State Hillary Clinton said Monday that the United States believes Iran's Revolutionary Guards are driving the country towards military dictatorship and should be targeted in any new UN sanctions, .

Speaking in Qatar before flying to Riyadh, Clinton denied the United States planned to attack Iran and said Washington wanted dialogue with Tehran but could not "stand idly by" while Iran pursued a suspected nuclear weapons program.

In remarks to Arab students at Carnegie Mellon's campus in Qatar, Clinton said the Revolutionary Guard Corps in Iran appears to have gained so much power, saying "the Revolutionary Guard ... we believe is, in effect, supplanting the government of Iran."

"That is how we see it. We see that the government of Iran, the supreme leader, the president, the parliament, is being supplanted and that Iran is moving toward a military dictatorship. That is our view."

Asked if Washington planned to attack Iran, she replied: "No, we are planning to try to bring the world community together in applying pressure to Iran through sanctions adopted by the United Nations that will be particularly aimed at those enterprises controlled by the Revolutionary Guard, which we believe is, in effect, supplanting the government of Iran."

Clinton has acknowledged that U.S. President Barack Obama's approach to Iran had not borne fruit, blaming Iran for refusing to engage and suggesting that a fourth UN Security Council sanctions resolution was the only option.

"What we are trying to do is to send a message to Iran, a very clear message, that we still would be open to engagement, we still believe that there is a different path for Iran to take," she said.

"But we want the world united in sending an unequivocal message to Iran that we will not stand idly by while you pursue a nuclear programme that can be used to threaten your neighbors and even beyond," she added.

While Arab states fear the possibility of Iran getting the bomb, and warn that it could spark a regional arms race, they are also uneasy about the possibility that military action by Israel against Iran could profoundly destabilise the region.

"We're still hoping that Iran will decide to forego any ... ambitions for a nuclear weapon," Clinton said. "But we cannot just keep hoping for that. We have to work to take action to try to convince the Iranian government not to pursue nuclear weapons."

Clinton's comments came a day after she declared that Iran was leaving the international community little choice but to exact a heavy price from Tehran over its provocative actions.

The White House announced Monday that Vice President Joe Biden and his wife, Jill, would visit the region the week of March 8, amid growing regional tensions over Iran's controversial nuclear program.

Biden will meet with key leaders, including Prime Minister Benjamin Netanyahu, Palestinian Authority President Mahmoud Abbas, Egyptian President Hosni Mubarak and Jordan's King Abdullah.

Referring to U.S.-led effort to force new sanctions on Iran over its nuclear program, Biden told Meet the Press on Sunday that he hoped to recruit China's support to the campaign.

"We already have the support of everyone from Russia to Europe, and I believe we could also garner China's support so to isolate [Iran]," Biden said, adding that the world had "to make it clear to them that we can't go on like this."

U.S. seems keen for Israeli show of restraint

On Sunday, Chairman of the Joint Chiefs of Staff Admiral Michael Mullen appeared keen to relay a public message to Israel: The U.S. is leading the international effort to levy harsh sanctions on Tehran, so Israel must exercise complete restraint.

Mullen told reporters he was concerned about the unintended consequences of a military strike on Iran's nuclear program.

The U.S. army chief said after arriving in Israel on Sunday that American policy on the matter is clear: "Iran must not acquire nuclear capability."

However, Mullen also said that if a regional confrontation were to break out following a strike on Iran, it "will be a big, big, big problem for all of us, and I worry a great deal about the unintended consequences of a strike."

In a fairly unusual step, Mullen held a short press conference at the U.S. Embassy in Tel Aviv. He then met with the Israel's military leadership, including Defense Minister Ehud Barak and Chief of Staff Gabi Ashkenazi.

In June 2008, when Mullen was last here, circumstances were similar: Then-president George Bush and his administration also interpreted Israeli statements as meaning that the country intended to attack Iran. Mullen was dispatched by the Bush administration in order to clarify that Israel cannot do this.

Mullen was asked Sunday about the red lines the Obama administration set for Iran's nuclear program. He refused to offer a detailed response, but said, "President Barack Obama was very clear that from a policy standpoint, Iran cannot have nuclear weapons."

He added that he still hoped a solution could be found through diplomacy and sanctions, and that there would not be a regional war.

"We haven't taken off any option from the table," he said. While the military option had not been discounted, "it's pretty hard to be specific."

He reiterated the assessment that unless Iran's nuclear program was halted, Tehran could have its first nuclear bomb within one to three years.

Mullen expressed concern at the behavior of the Iranian leadership and said it had a destabilizing influence on the region. He cited as cause for concern Iranian President Mahmoud Ahmadinejad's recent statement that Iran could step up uranium enrichment, and added that the country was linked to Hezbollah, Hamas and the instability in Yemen, and played a role in Afghanistan.

Stressing America was committed to Israel's security, he commended the countries' close defense and security ties, and their stabilizing effect on the region.

The admiral also noted that the U.S. has taken steps to protect several countries in the region from Iranian threats, and mentioned that Patriot air defense missiles had been deployed in the United Arab Emirates. Mullen added that all measures are defensive.