The disagreement had been on the back burner since talks began February 18 on a comprehensive deal meant to constrain Iranian nuclear work that can make such arms in exchange for full sanctions relief on Tehran's economy. Negotiators had concentrated on less contentious components of a final deal in previous rounds.
But both sides reported difficulties as the talks went into their third day, and Washington said it was up to the Iranians to make concessions.
"Significant gaps remain," the official, who wasn't authorized to speak publicly about confidential information, said on condition of anonymity. "Iran still has some hard decisions to make."
A Western official with detailed knowledge of the negotiations was more specific, telling The Associated Press the two sides were at loggerheads over enrichment.
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Iran denies any interest in nuclear weapons, saying its enrichment program is meant only to make reactor fuel. But because the technology can also create weapons-grade uranium for warheads depending on the level of enrichment, Washington and its allies want strict constraints on its size and scope.
But general differences have long been known. Iran's nuclear chief, Ali Akbar Salehi, has said publicly that Tehran needs up to 100,000 centrifuges, the enriching machines, for a future nuclear network.
That's about five times as many as the centrifuges Iran now has standing but idle, 10 times that of the machines actually enriching, and much more than the few thousand that diplomats say the US and its allies are prepared to allow.