In a speech delivered to a group of teachers in Tehran, Mr. Khamenei, who has final say over most matters of state in Iran, said he didn’t approve of any negotiations “under the ghost of a threat,” according to his official website.

Mr. Khamenei said two U.S. officials had threatened a military attack against Iran recently, but he didn’t elaborate and it wasn’t immediately clear what he was referring to.

Mr. Khamenei has expressed doubts about the talks before. His position is being carefully watched because his word could scuttle a final deal between Iran and the so-called P5+1, a group of six world powers including the U.S., China, Russia, France, Germany and the U.K.