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

Obama was asked in an impromtu 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.

"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."

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