Iran has begun producing enriched uranium at 60 per cent purity at the country's underground Fordo nuclear plant, official media reported on Tuesday, describing it as a response to a resolution by the United Nations' nuclear watchdog.
The step was seen as a significant addition to the country's nuclear programme.
Enrichment to 60 per cent purity is one short, technical step away from weapons-grade levels of 90 per cent.
Nonproliferation experts have warned in recent months that Iran now has enough 60 per cent-enriched uranium to reprocess into fuel for at least one nuclear bomb.
Iran is already enriching to 60 per cent purity at its Natanz nuclear facility in central Iran. Fordo is some 100 kilometres (62 miles) south of the capital of Tehran.