The United States warned today that Iran's continued uranium enrichment meant that any new offer by world powers on its nuclear programme would be more burdensome than one it had already rejected. White House spokesman Robert Gibbs said increased expectations required for any deal reflected the fact that Iran's enriched uranium stocks were now larger than they were when previous talks broke down last year. Gibbs spoke after the New York Times reported that the Obama Administration and its European allies were preparing a new, more onerous offer for Iran than the one rejected by supreme leader Ayatollah Ali Khamenei last year.