Iran sees "no more room" for negotiations on the design of its Arak reactor, Tehran's nuclear chief said, refusing to give ground on a key issue in international negotiations.
Western nations fear the unfinished reactor could provide Iran with weapons-grade plutonium, but Tehran insists it is solely for research purposes.
Iran has already said it will make some modifications to the heavy water reactor, located 240 kilometres (145 miles) southwest of Tehran, and limit plutonium output.
The United States has proposed transforming Arak into a light water reactor so that it produces far less plutonium, but Tehran has refused.
"On Arak, we have said we were ready to design it so that the concerns are lifted. This matter is settled to some extent on the technical aspect and there is no more room for further negotiations," Ali Akbar Salehi was quoted Thursday by media as saying.
The future of the site is one of the main focuses of talks between Iran and six world powers under way in Vienna aimed at striking a deal over Tehran's nuclear programme by a November 24 deadline.
Iran denies seeking to build an atomic bomb and says it wants to expand its nuclear programme in order to generate electricity and help cancer patients.