WASHINGTON - US President Barack Obama warned Iran on Tuesday that he would isolate Tehran with a "significant regime of sanctions" over its nuclear proggramme.
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 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.
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, as it currently imports 40% of its gasoline needs.
"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.
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.
The loudest call for sanctions came from Israel where hardline Prime Minister Benjamin Netanyahu called for "crippling" measures against Tehran.
Israel is the only country in the Middle East that actually has nuclear bombs.
Sanctions on Iran would be ineffective, Brazil said Tuesday, suggesting instead that UN atomic energy agency chief Yukiya Amano should go to Tehran to address problems over its nuclear program.
"We don't believe that sanctions will prove effective. Truly, Iran is a big country and a large and diverse economy, even if they (sanctions) could cause some problems," Brazilian Foreign Minister Celso Amorim told reporters.
"What is needed is an initiative, maybe involving the IAEA (International Atomic Energy Agency) director general," he said.
Amorim said a proposal by Iranian President Mahmoud Ahmadinejad for Iran to exchange its uranium for already enriched uranium should be weighed.
"I think the (IAEA) director should go (to Iran) to analyze how this could work, have a look at what difficulties exist. I don't believe this has been done," he said.
Brazil enjoys good relations with Iran, and hosted a visit by Ahmadinejad last year. Brazilian President Luiz Inacio Lula da Silva is to travel to Iran in May.
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