Since January, Iran has begun enriching uranium at up to 20 percent purity, a technical step away from weapons-grade levels.
Iran had previously committed to suspending operations at the plant as part of the Iran Nuclear Deal four years ago, but following Washington's abandonment of the deal, Iran has started abandoning its own commitments as well.
Iran's President Hassan Rouhani said Tehran would begin injecting gas into centrifuges at the Fordow plant, a restriction it had originally accepted in return for lifting international sanctions.
Iran's uranium enrichment level has passed 4.5 percent, exceeding the cap set by the 2015 nuclear deal.
A senior advisor to Iran's supreme leader says Tehran could boost its uranium enrichment to "five percent" even as a top Iranian cleric warns Britain should be "scared' of Tehran's response over oil tanker capture in Gibraltar.
Iran has exceeded the upper limit of its low-enriched uranium reserves for the first time after the US withdrew from the accord last year. The UN's atomic watchdog also confirmed the breach in a report.
A spokesman for Iran's atomic agency said Tehran would increase uranium enrichment levels "based on the country's needs".
Last week, Iran notified China, France, Germany, Russia and the UK of its decision to halt some commitments under the 2015 nuclear deal, a year after the US unilaterally withdrew from the accord and re-imposed sanctions.
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