Iran's nuclear chief on Thursday ruled out further negotiations on the design of a reactor that the West fears could be used to produce plutonium for an atomic bomb.

The future of the Arak reactor is one of the main issues in talks between Iran and six world powers under way in Vienna aimed at striking a deal on Tehran's nuclear programme by November 24.

Iran insists the unfinished reactor, located 240 kilometres (145 miles) southwest of Tehran, is solely for research purposes.

It has already promised to make some modifications to the design of the heavy water reactor to 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 by local media as saying.

Under an interim agreement with world powers which came into effect in January, Iran agreed to scale back part of its nuclear programme in return for a partial lifting of Western sanctions.

Tehran 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.

Iranian officials have said the design of the Arak reactor -- whose construction is being supervised by the UN nuclear watchdog -- will be modified to produce one kilogramme (2.2 pounds) of plutonium a year instead of eight kilos as originally planned.

Salehi reaffirmed Iran's position that its uranium enrichment capacity must be 190,000 SWU (Separative Work Units) -- equivalent to 190,000 first-generation centrifuges -- close to 20 times its current processing ability.

The West wants a drastic reduction in the number of centrifuges.

"We need output of at least 190,000 SWU within the next eight years" to provide fuel for a power plant in the southern Gulf port city of Bushehr, and for a research reactor in Tehran, Salehi said.

Earlier this month, Russia signed a contract with Iran to build two new reactors at the Russian-built Bushehr plant.

There is also the possibility of Moscow transferring some sensitive technology relating to the production of fuel rod components.

Salehi, who heads the Atomic Energy Organisation of Iran, also dismissed the idea of moving fuel manufactured in Iran abroad.

"It makes no sense to make so much effort here to produce fuel and then send it overseas to be stored," he said.

He also said Iran would refuse a "special" inspection system for its nuclear sites.

The West wants increased surveillance of the sites by the International Atomic Energy Agency, the UN watchdog, to ensure there is no illegal activity.