The head of the UN nuclear watchdog has urged the US to become more engaged in attempts to resolve a dispute about Iran's nuclear programme.
Tehran has rejected US offers of incentives to stop its programme
IAEA director general Mohamed ElBaradei said Washington would have to step in as Tehran needed security guarantees beyond those given by the EU.
President George W Bush announced 10 days ago that the US was prepared to extend economic incentives to Iran.
Iran said at the time that the offer was "insignificant".
Speaking at a nuclear energy conference in Paris on Monday, Mr ElBaradei said that regional security was "not only a European affair".
"I think that when the issue of security comes up, obviously the US will have to step in, because... the Iranians will very much need the American and not just European assurance," he said.
Washington accuses Iran of using its nuclear enrichment programme as a cover for developing nuclear weapons.
Tehran denies seeking nuclear weapons, but has suspended uranium enrichment after negotiations with France, Germany and the UK.
The US and European Union want that move made permanent, and have threatened to seek UN sanctions if Iran does not comply.