Russia and Iran have signed a landmark agreement for Moscow to supply Tehran with fuel for its first nuclear reactor.

The signing ceremony Sunday at Iran's Bushehr nuclear power plant, which Russia helped build, had been delayed 24 hours as talks continued.

European and U.S. officials have expressed concern that spent fuel from the reactor could be enriched and used to create nuclear weapons. Diplomats say there is evidence that Iran has known since the late 1980s how to enrich uranium. They say Iran obtained the information from a black market network operated by Pakistan's Abdul Qadeer Khan.

Iran says the purpose of its nuclear program is the generation of electricity, not the development of nuclear weapons.

The new agreement calls for all spent nuclear fuel to be returned to Russia to ease concerns that Iran might reprocess it for use in nuclear weapons.