"Time is running out. That deadline is the end of the year," White House spokesman Robert Gibbs told reporters in a news briefing.
Iran has resisted a deal with Western powers that would see its low-enriched uranium sent abroad for processing into uranium for making fuel. Gibbs said it was "pretty clear" that the Iranians were backing out of that agreement.
Washington and its allies suspect Iran is secretly using its nuclear energy program to develop nuclear weapons, but Tehran says it is enriching uranium only to generate electricity.
(Reporting by Matt Spetalnick, editing by Vicki Allen)
