The US leader also looked ahead to the diplomatic chess game involved in toughening sanctions against the Islamic Republic, praising Russia for its firm response but expressing uncertainty on China's stance.

Obama was asked in an impromptu appearance in the White House briefing room how quickly six key world powers were moving to toughen a regime against Tehran given Iran's enrichment activity.

"It's moving along fairly quickly," Obama said.

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 proposal by the 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.

"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.

With such concerns in mind, Obama said the world must be prepared to resort 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.

He gave no details about the regime and appeared uncertain as to whether China would support tougher sanctions.

"We are confident right now that the international community is unified around Iran's misbehavior in this area," Obama said.

"How China operates at the (UN) Security Council as we pursue sanctions is one thing we will have to see," said the president.

"But one thing I am pleased to see is how forward-leaning the Russians are on this issue."

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