The council's 15 members voted Saturday to expand UN punitive measures imposed on the Islamic Republic in December after it repeatedly refused to halt uranium enrichment, which major powers suspect could be a cover to develop nuclear arms.

France's UN Ambassador Jean-Marc de La Sabliere said the council's united stand left Tehran with "a very clear choice: either suspend enrichment and then the council suspends its measures or increase its isolation . . . We hope Iran will make the right choice."

In Washington, US Undersecretary of State Nicholas Burns hailed the vote as "a significant international rebuke to Iran and a significant tightening of the international pressure" on the Islamic Republic.

"The international community is united, it's time for Iran to comply or potentially face harsher measures in the future," said Gordon Johndroe, spokesman for the White House's National Security Council.

The six powers trying to rein in Tehran's nuclear ambitions -- Britain, China, France, Germany, Russia and the United States -- also urged a negotiated way out of the crisis.

In a statement, their foreign ministers proposed "further talks with the Islamic Republic of Iran to see if a mutually acceptable way can be found to open negotiations."

Reiterating that the package of economic, diplomatic and security incentives they offered Tehran last June was still on the table, the statement read: "We urge Iran to take this opportunity to engage with us all and to find a negotiated way forward. Our proposals would bring far-reaching benefits to Iran and to the region."

Burns said European Union foreign policy chief Javier Solana and other EU diplomats would contact the Iranian government "in the days and weeks ahead to see if they might reconsider their obstinate refusal to renegotiate."

There was no early sign that Iran was willing to back down.

Iranian Foreign Minister Manouchehr Mottaki swiftly dismissed the UN sanctions as "unlawful" and "unjustifiable actions" orchestrated by Western members of the council.

"This is the fourth time in the last 12 months that in an unwarranted move orchestrated by a few of its permanent members, the Security Council is being abused to take an unlawful, unnecessary and unjustifiable action against the peaceful nuclear program of the Islamic Republic of Iran," he told the Council after the vote.

"As we have stressed time time again, Iran's nuclear program is completely peaceful," Mottaki said. "We have expressed our readiness, taken unprecedented steps and offered several serious proposals to address and ally any possible concern."

Russia, which along with China has energy interests in Iran but has recently taken a tougher line against Tehran, urged the Iranians to respond to the offer of the six powers.

"The cumulative effect of the unanimous adoption of the resolution and the strong positive signal of the six foreign ministers should be properly appreciated and responded to by Iranian authorities," Russian Ambassador Vitaly Churkin said.

Iran had hoped for stronger support within the Council from fellow non-aligned nations such as South Africa, and fellow Muslim countries such as Indonesia and Qatar.

But despite their reservations about the resolution, and their sympathy for Iran's argument that it has the right to enrich uranium for peaceful purposes, South Africa, Indonesia and Qatar voted in favor because of the "lack of international confidence" in the nature of the Iranian nuclear program.

Iran has repeatedly denied allegations that it is trying to develop nuclear weapons, saying its uranium enrichment is purely to generate electricity. Enriched uranium can provide the raw material for both military and civilian nuclear use.

The new resolution, agreed after days of closed-door bargaining, blocks all Iranian arms exports and freezes the overseas assets of 28 additional officials and institutions linked to Iran's nuclear and ballistic missile programs.

It also restricts financial aid or loans to Tehran, and sets a fresh 60-day deadline for Iran to comply with UN demands or face "further appropriate measures" -- economic sanctions but no military action -- under Article 41 of the UN Charter.

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