Iran's president says his country is installing more advanced centrifuges at its main uranium enrichment facility, a step certain to add to international concern over Tehran's accelerating nuclear program.

Iran announced a week ago that it was enriching uranium to a higher level for use in fueling a reactor that produces medical isotopes. The new enrichment work is of major concern to the U.S. and its allies because at higher levels the material can be used in nuclear warheads.

President Mahmoud Ahmadinejad told a news conference the new centrifuges are not yet operational but are five times more efficient than an earlier model at its Natanz enrichment plant.

Iran says its nuclear work is only for peaceful purposes.