The United States has warned Iran that "time and patience is running out" over its controversial nuclear program.
White House spokesman Robert Gibbs reiterated U.S. warnings of "consequences" if Iran continues to develop uranium enrichment capabilities.
Tehran on February 23 submitted a letter to the International Atomic Energy Agency (IAEA) with its first official reply to an IAEA-brokered fuel-swap proposal.
The letter said Iran was prepared to buy fuel for a nuclear reactor or swap its own stockpile of low-enriched uranium for the fuel, provided the swap took place on its own territory.
Western powers have offered to make such a trade only outside Iran. U.S. State Department spokesman Philip Crowley also called the proposal "unacceptable."
On February 22, Tehran said it was considering building two more uranium-enrichment plants concealed inside mountains to protect them from air strikes.
Iran insists that its nuclear activities are peaceful.
But an IAEA report prepared last week says the agency is worried Iran may currently be working on making a nuclear warhead -- suggesting for the first time that Tehran had either resumed such work or never stopped when U.S. intelligence thought it did.
compiled from agency reports