Khamenei, the highest authority in the Islamic Republic, already said last month there would be no more talks with the United States after the nuclear deal, but has not previously declared an outright ban.
His statements directly contradict those of moderate Iranian President Hassan Rouhani, who says his government is ready to hold talks with the United States on how to resolve the conflict in Syria, where the two countries back opposing sides.
“Negotiations with the United States open gates to their economic, cultural, political and security influence. Even during the nuclear negotiations they tried to harm our national interests.,” Khamenei was quoted as saying on his website.
“Our negotiators were vigilant but the Americans took advantage of a few chances,” he said.