TEHRAN, Iran (AP) — Iran has begun producing enriched uranium at 60% purity at the country's underground Fordo nuclear plant, official media reported 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 program.

Enrichment to 60% purity is one short, technical step away from weapons-grade levels of 90%. Nonproliferation experts have warned in recent months that Iran now has enough 60%-enriched uranium to reprocess into fuel for at least one nuclear bomb.

Iran is already enriching to 60% purity at its Natanz nuclear facility in central Iran. Fordo is some 100 kilometers (62 miles) south of the capital of Tehran.