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The officials spoke only on condition of anonymity because they were not authorized to discuss details of the sensitive negotiations as the latest round of talks began between U.S. Secretary of State John Kerry and Iranian Foreign Minister Mohammed Javad Zarif. The negotiators are racing to meet an end-of-March deadline to reach an outline of an agreement that would grant Iran relief from international sanctions in exchange for curbing its nuclear program. The deadline for a final agreement is June 30.

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One senior U.S. official declined to comment on the specific proposal but said the goal since the beginning of the talks has been “to have Fordo converted so it’s not being used to enrich uranium.” That official would not say more.

The officials stressed that the potential compromise on Fordo is just one of several options on a menu of highly technical equations being discussed in the talks. All of the options are designed to keep Iran at least a year away from producing an atomic weapon for the life of the agreement, which will run for at least 10 years. U.S. Energy Secretary Ernest Moniz has joined the last several rounds as the negotiations have gotten more technical.

Experts say the compromise for Fordo could still be problematic. They note it would allow Iran to keep intact technology that could be quickly repurposed for uranium enrichment at a sensitive facility that the U.S. and its allies originally wanted stripped of all such machines — centrifuges that can spin uranium gas into uses ranging from reactor fuel to weapons-grade material.