"Iran's proposal today does not appear to be consistent with the fair and balanced draft agreement proposed by the IAEA (International Atomic Energy Agency) in consultation with the United States, Russia and France," the official said on condition of anonymity.
Iran's foreign minister earlier proposed that Tehran swap 880 pounds (400 kilograms) of low-enriched uranium for nuclear fuel in an exchange on the island of Kish, a free trade zone, as the first phase of a deal with world powers.
The US official said Iran's offer contained "nothing new" and urged the country to take up the existing IAEA proposal, which calls on Iran to send 2,645 pounds (1,200 kilograms) of its low enriched uranium to Russia "in one batch."
"We remain committed to these terms. Unfortunately, Iran has been unwilling to engage in further talks on its nuclear program," the official said. "We urge Iran not to squander this opportunity."
The IAEA had previously ruled out a swap taking place inside Iran.
"I don't think that is an option. The whole purpose of the deal is to defuse the crisis," outgoing chief Mohamed ElBaradei said last month at the agency's Vienna base before handing over to his successor, Yukiya Amano.
Many in the West suspect Iran is developing technology to enrich uranium to highly refined levels to covertly build a nuclear bomb, a charge Tehran vehemently denies, saying its nuclear program serves peaceful purposes.
The US official said the IAEA-brokered agreement was "an opportunity for Iran to begin to build confidence in the peaceful nature of its nuclear program."
On Friday, the White House warned Iran it faced "credible consequences" if it did not respond positively to the offer by the end of the year, while European Union leaders warned that Tehran's refusal to negotiate over its nuclear program must be met with a tough response.
