Negotiations with Iran over limiting its nuclear program appeared close to agreement Thursday, but the reported outlines of a possible deal contained concessions by international negotiators expected to make it politically unpalatable in Washington.

The Associated Press reported that the United States is considering letting Iran run several hundred centrifuges used to enrich uranium at Fordow, a heavily fortified underground site near the Shiite holy city of Qom that Iran kept secret from international inspectors until it disclosed the facility in 2009. In exchange, Iran would reduce the number of centrifuges it operates elsewhere and would use those at Fordow to enrich elements other than uranium.