Iran has resumed enriching uranium to 20% purity, in its most significant breach yet of its 2015 deal with world powers to end nuclear sanctions. The global nuclear watchdog confirmed the process had begun at the underground Fordo plant on Monday. Enriched uranium can be used to make reactor fuel but also nuclear bombs. Weapons-grade uranium is 90% purity. A US state department spokesperson accused Iran of pursuing a "campaign of nuclear extortion". Iran, which insists that its nuclear programme is peaceful, has rolled back a number of commitments under the deal. It has said it is retaliating for the US economic sanctions that were reinstated in 2018 by President Donald Trump when he abandoned the accord, also known as the Joint Comprehensive Plan of Action (JCPOA). In a separate development, Iran's Revolutionary Guards said they had seized a South Korean-flagged chemical tanker in the Gulf "due to the repeated infringement of maritime environmental laws". South Korea's foreign ministry demanded its immediate release of the Hankuk Chemi and said a South Korean anti-piracy unit stationed in the Strait of Hormuz had been dispatched to the area.