"It (enrichment) is not stopped, rather it is suspended and this will not last long. Whenever we think it is right we will restart it," the mid-ranking cleric told a students gathering, reported by the official IRNA news agency.

Western diplomats have told Reuters Iran has been acquiring large amounts of equipment for centrifuges used to enrich uranium despite its promise to suspend all activities.

Enriched uranium can be used to fuel nuclear power stations for domestic electricity supply. But a further enrichment to so-called weapons grade can be deployed in warheads.

Iran is building a nuclear power station at the southern port of Bushehr with Russian assistance and has ordered plans drawn up for a second facility.

It insists its nuclear programme is geared towards meeting booming domestic electricity demand and freeing up its finite fossil fuels for export.

Although Iran last month signed the Additional Protocol to the nuclear Non-Proliferation Treaty (NPT) that allows snap U.N. inspections of nuclear facilities, pressure on Iran to come clean on its activities is still fierce.

Iranian Foreign Minister Kamal Kharrazi has said that under the NPT, Iran was entitled to pursue nuclear activities for peaceful purposes. "We have suspended the activities of uranium enrichment, but this does not mean we are going to stop it forever," he said.

Mohamed ElBaradei, head of the U.N. nuclear watchdog, said last week Iran would face "serious implications" if it did not fully co-operate with efforts to monitor its nuclear programme.

The father of Pakistan's nuclear bomb Abdul Qadeer Khan is a key suspect in an investigation into the sale of nuclear technology to Iran, Pakistani officials said on Monday.