But Burns said a clean bill of health from the IAEA alone would not be enough to halt steps to stiffer economic sanctions.

“Our judgment is that if Iran has not suspended in the next couple of weeks, that’s not sufficient, it will remain a refusal to meet Security Council requirements. That will be a highly relevant factor for us,” he said.

“Our hope is the following: first, a third sanctions resolution will be passed as soon as possible. Second, we’d very much support seeing the EU go forward with (its own) sanctions. Third, major trading partners of Iran should reduce trade to show Iran that this is not business as usual.”

Iran warned the United States on Wednesday it would find itself in a “quagmire deeper than Iraq” if it attacked the Islamic state.

US President George W Bush has suggested a nuclear-armed Iran could lead to World War Three. Washington says it wants a diplomatic solution but Burns said on Wednesday more “tough-minded diplomacy” was needed to make that work.

He said this should include European sanctions on Iran, which some large EU members are reluctant to pursue.