Iran's Supreme Leader Ayatollah Ali Khamenei is prohibiting any further negotations with the United States.
His ban puts the brakes on moderates hoping to end Iran's isolation after reaching a nuclear deal with world powers in July.
In his address to Revolutionary Guards Navy commanders, Khamenei said talks with the U.S. brought only disadvantages to Iran.
SOUNDBITE: (Farsi) IRAN'S SUPREME LEADER, AYATOLLAH ALI KHAMENEI, SAYING:
"Negotiation with the U.S. is forbidden, because of the numerous disadvantages that it has and the benefits that it doesn't have, this is different to negotiating with a state which has no such facilities and no such motivation (against Iran)."
His statements directly contradict those of moderate Iranian President Hassan Rouhani, who says his government is ready to talk with the U.S. on how to resolve the conflict in Syria.
Khamenei supported the last 18 months of negotiations on nuclear talks but has not publicly endorsed the deal itself.
The agreement curbs Tehran's nuclear program in exchange for lifting crippling Western sanctions on Iran.