WASHINGTON: 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 per cent, 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-per cent enriched uranium abroad so that it can be further upgraded to fuel the reactor. Experts say once Iran enriches uranium to 20 per cent, it can proceed to the 93 per cent 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 weaponisation, and that is not acceptable to the international community,” Obama said. “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 per cent 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.