US President Barack Obama warned Iran on Tuesday that he would isolate the Islamic republic with a "significant regime of sanctions" if it continued to set its stall on developing nuclear weapons.

Stepping up the heat on Iran's leaders at an impromptu appearance in the White House briefing room, Obama said the six key world powers were "moving along fairly quickly" to toughen measures against Tehran.

He indicated that his administration had made headway in persuading Russia to overcome its traditional resistance to imposing new sanctions on Iran, even if there was still some opposition from China.

"It's moving along fairly quickly," Obama said, referring to negotiations on what to do next involving six leading world powers -- the United States, Russia, China, France, Britain and Germany.

Iran announced on Tuesday it has begun work to enrich uranium to 20 percent, which it says is for a medical research reactor in Tehran.

The move suggested Iran was spurning a four-month-old proposal by the UN's International Atomic Energy Agency to ship most of its stocks of 3.5-percent enriched uranium abroad so that it can be further upgraded to fuel the reactor.

Experts say that once Iran enriches uranium to 20 percent, it can proceed to the 93 percent needed to produce nuclear weapons since the technology is the same. Iran maintains the enrichment is purely for civilian energy purposes.

"Despite the posturing that the nuclear power is only for civilian use... they in fact continue to pursue a course that would lead to weaponization, and that is not acceptable to the international community," Obama said.

After months trying to engage Iranian leaders and persuade them to accept the UN-brokered deal to defuse the crisis, Obama said the world must be prepared to pressure Iran to change course, even if the "door is still open" to negotiations.

"What we are going to be working on over the next several weeks is developing a significant regime of sanctions that will indicate to them how isolated they are from the international community as a whole," Obama said.

In Moscow, the powerful head of Russia's national security council, Nikolai Patrushev, said Tehran's announcement that it had started work to produce 20 percent enriched uranium cast doubt on its claims not to be pursuing weapons.

Patrushev indicated the Kremlin's patience in trying to seek dialogue with Tehran was wearing thin.

"Political and diplomatic methods are important for regulating, but everything has its limit," Patrushev was quoted as saying by Russian state news agencies.

His comments were an unusual expression of concern from Moscow, which has long said there was no evidence that Iran was pursuing anything other than a civilian nuclear energy program.

China was alone among the six powers in calling for more talks to resolve the impasse.

"We hope the relevant parties will exchange views on the draft deal on the Tehran research reactor and reach common ground at an early date which will help solve the issue," Chinese foreign ministry spokesman Ma Zhaoxu said.

Ahmet Davutoglu, the foreign minister of Turkey, a country with good ties with both Iran and the West, is to visit Iran next week to push for a diplomatic solution to the stand-off.

Washington has called the latest Iranian moves "provocative" and along with France is pressing ahead with moves at the UN Security Council for a fourth sanctions resolution against Iran.

US Defense Secretary Robert Gates "thinks this is a matter of weeks, not months," Pentagon spokesman Geoff Morrell told reporters in Paris, where Gates met French President Nicolas Sarkozy.

State Department spokesman Philip Crowley said the Iranian decision to further enrich uranium "puts us in a much stronger position" to move ahead with plans to "pressure" the Iranian government rather than its people.

US officials have discussed sanctions targeted at Iran's Revolutionary Guard, which is responsible for the country's nuclear program and is behind the crackdown on anti-government protests there.

"Iran is increasingly isolated," Crowley told reporters in Washington.

The loudest call for sanctions came from Israel where Prime Minister Benjamin Netanyahu called for "crippling" measures against Tehran during a gathering of European Union ambassadors in Jerusalem.