US President Donald Trump says he will withdraw the US from an Obama-era nuclear agreement with Iran.

In response, Iran said it was preparing to restart uranium enrichment, key for making both nuclear energy and weapons.

The so-called Joint Comprehensive Plan of Action curbed Iran’s nuclear activities in return for the lifting of sanctions that had been imposed by the UN, US and EU. Mr Trump had previously complained that the deal only limited Iran’s nuclear activities for a fixed period; had failed to stop the development of ballistic missiles; and had handed Iran a $100bn windfall that it used “As a slush fund for weapons, terror, and oppression” across the Middle East.

US National Security Advisor John Bolton is reported as saying that European companies doing business with Iran will have to finish within six months or face US sanctions.

With a stroke of his pen President Trump has jeopardised the one agreement – good or bad -that seeks to constrain Iran’s nuclear ambitions.

“At a time when we are all rooting for diplomacy with North Korea to succeed, walking away from the JCPOA risks losing a deal that accomplishes – with Iran – the very outcome that we are pursuing with the North Koreans,” he said.

Saudi Arabia, Iran’s regional rival, says it “Supports and welcomes” Mr Trump’s moves towards pulling out of the deal.

The so-called Joint Comprehensive Plan of Action saw Iran agree to limit the size of its stockpile of enriched uranium – which is used to make reactor fuel, but also nuclear weapons – for 15 years and the number of centrifuges installed to enrich uranium for 10 years.

Iran insists its nuclear programme is entirely peaceful, and its compliance with the deal has been verified by the International Atomic Energy Agency.