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.

Iran Must Cooperate With Uranium Probe: IAEA Resolution

The United Nations nuclear watchdog's 35-nation Board of Governors passed a resolution ordering Iran to cooperate urgently with the agency's investigation into uranium traces found at three undeclared sites.

The resolution drafted by the United States, Britain, France and Germany says "it is essential and urgent" that Iran explain the origin of the uranium particles and more generally give the International Atomic Energy Agency all the answers it requires.

While it was not the first resolution the board has passed against Iran on the issue — another was adopted in June — its wording was stronger and hinted at a future diplomatic escalation.

"Iran must now provide the necessary cooperation, no more empty promises," the United States said in its statement to the board shortly before the resolution was adopted with 26 votes in favor, five abstentions and two countries absent, according to diplomats in the meeting. Only Russia and China voted against adoption. (KB/VOA)