A British newspaper said on Monday that Iran is working on testing a final component to build a nuclear bomb.
The Times newspaper claims to have obtained confidential intelligence documents showing that a four-year plan to test a neutron initiator, the part of an atomic bomb that sets off an explosion.
Foreign intelligence agencies date them to early 2007, four years after Iran was thought to have suspended its weapons program, the paper said.
The Times said independent experts have confirmed that the neutron source mentioned in the documents has no possible use other than in a nuclear warhead.
Iran, the world's fifth-largest crude oil exporter, says its nuclear program is aimed at generating electricity so that it can export more of its gas and oil.
The Times said intelligence on the alleged neutron initiator work had been passed on to the U.N. nuclear watchdog, which has been investigating suspicions of Iranian attempts to weaponize the uranium enrichment process for five years.
Iran which is currently under three sets of UN sanctions for refusing to halt uranium enrichment is set to launch its first civilian nuclear power plant in the next few months but denies that its controversial atomic energy program has military aims.