The United Nations Security Council votes unanimously Saturday to expand sanctions against Iran at the United Nations headquarters. Moderately tougher sanctions were imposed against Iran because it refused to stop enriching uranium.

United Nations ? The U.N. Security Council unanimously voted Saturday to impose additional sanctions against Iran for its refusal to stop enriching uranium – a move intended to show Tehran that defiance will leave it increasingly isolated.

Iran immediately rejected the sanctions and said it had no intention of suspending its enrichment program, prompting the United States to warn of even tougher penalties.

“The world must know – and it does – that even the harshest political and economic sanctions or other threats are far too weak to coerce the Iranian nation to retreat from their legal and legitimate demands,” Iranian Foreign Minister Manouchehr Mottaki told the Security Council after the vote. “Suspension is neither an option nor a solution.”

The moderately tougher sanctions include banning Iranian arms exports, and freezing the assets of 28 people and organizations involved in Iran’s nuclear and missile programs.

About a third of those are linked to the Revolutionary Guard, an elite military corps.

“It’s a significant international rebuke to Iran and it’s a significant tightening of international pressure on Iran,” said Nicholas Burns, undersecretary for political affairs at the State Department. If Iran does not comply, “there’s no question” that the United States will seek a third and tougher resolution, he added.

In December, the 15-member Security Council ordered all countries to stop supplying Iran with materials and technology that could contribute to its nuclear and missile programs. It also ordered a freeze on assets of 10 key Iranian companies and 12 individuals related to those programs. Iran responded by expanding enrichment, a process that can be used to produce nuclear energy or nuclear weapons.

The United States and some of its allies fear Iran’s nuclear program is a cover for producing atomic weapons, but Iran insists it is only for energy production.

Mottaki said Iran would return to negotiations over its nuclear program only if the United States and its European allies dropped the “unfair and unacceptable precondition” that it first suspend uranium enrichment.

But world powers held out hope that Iran would back down before the dispute escalated even further.

The European Union’s foreign policy chief, Javier Solana, said nations involved in the dispute had tasked him to resume contacts with Iranian negotiator Ali Larijani “to see whether we can find a route to negotiations.”

“The door to negotiations is open; I hope we can together find a way to go through it,” Solana said in statement.

Burns said that because of a “tumultuous political environment” in Iran, “we believe there is a faction inside that government that wishes to accept this offer to negotiate.”

He was apparently referring to criticism from some in Iran that President Mahmoud Ahmadinejad’s confrontational rhetoric has deepened the country’s isolation.

Ahmadinejad canceled a planned appearance before the Security Council and sent Mottaki instead, claiming the U.S. failed to deliver his visa in time. The U.S. said it had issued the visa promptly.

Raising tensions, Iran detained 15 British sailors and marines Friday in what it said were Iranian territorial waters near Iraq. The 15 had been on a mission to search for smugglers in Iraqi waters.

The six world powers that drafted the new resolution spent Friday trying to overcome objections from several council members, reflecting concerns that anything short of consensus would weaken efforts to rein in Iran’s nuclear defiance.

There were several minor concessions but no changes to the key sanctions agreed upon last week by the United States, China, Russia, Britain, France and Germany.

The new sanctions – already a compromise – are considered modest.