The US has dismissed a call by the head of the UN's nuclear agency to offer Iran security assurances as a boost to talks over its nuclear programme.
Tehran has rejected US offers of incentives to stop its programme
State department spokesman Adam Ereli said the issue was not what the US would do, but what Iran would do, to allay the international concerns.
Iran is said to be seeking assurances of US and Israeli non-aggression.
The director of the UN's nuclear agency, Mohammed ElBaradei, denied that the talks were in crisis.
Speaking at a nuclear energy conference in Paris on Monday, Mr ElBaradei, head of the International Agency for Atomic Energy (IAEA), expressed confidence that they would result in a successful outcome.
He said he hoped the conclusion of the negotiations between three European states and Tehran would allow Iran to use nuclear energy, while satisfying the international community that Iran's nuclear programme is solely for peaceful purposes.
US 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".
Mr ElBaradei said on Monday 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.