Iran has started the process of enriching uranium to 20% in defiance of the West, Iranian state media says.
The process was begun at the Natanz plant in the presence of International Atomic Energy Agency (IAEA) inspectors, al-Alam state television reported.
The move came after Western powers stepped up pressure for international sanctions against Iran.
The US and its Western allies say Iran is trying to develop a nuclear weapon – a charge Iran denies.
Iran currently enriches uranium to a level of 3.5% but requires 20% enriched uranium for its Tehran research reactor, which is meant to produce medical isotopes. A bomb would require uranium enriched to at least 90%.
Enriching to 20% would take about one year, using the 2,000 centrifuges at the underground Natanz facility, experts say.
But moving from 20% to 90% would take only six months and only require between 500 and 1,000 centrifuges.
Iran insists its nuclear programme is entirely peaceful and designed to provide civilian power.
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