increasing enrichment to just short of bomb grade in small quantities and barring international inspectors from key sites in late February — Mr. Zarif insists that these moves are easily reversible.

American intelligence officials say that while Iran has bolstered its production of nuclear material — and is probably only months from being able to produce enough highly enriched uranium for one or two bombs — even now, there is no evidence Iran is advancing on its work to fashion a warhead. “We continue to assess that Iran is not currently undertaking the key nuclear weapons-development activities that we judge would be necessary to produce a nuclear device,” Avril D. Haines, the director of national intelligence, said in a report last month.

scandal over Mr. Zarif, whose criticism of internal decision-making recently leaked, apparently in an effort to damage his reputation and any chance he had to run for the presidency.

Ayatollah Khamenei refuted the criticism without naming Mr. Zarif, but he said the comments were “a big mistake that must not be made by an official of the Islamic Republic” and “a repetition of what Iran’s enemies say.”

At the same time, by downplaying Mr. Zarif’s role, the supreme leader reaffirmed his support for the talks while also sheltering them from criticism by hard-liners, said Ellie Geranmayeh of the European Council on Foreign Relations.

Steven Erlanger reported from Brussels, and David E. Sanger from Washington. Farnaz Fassihi contributed reporting from New York.

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