Iran has cancelled the accreditation of an International Atomic Energy Agency (IAEA) inspector who was prevented from entering a nuclear facility last week. It said the inspector triggered an alarm at the gate to the Natanz uranium enrichment plant, raising fears that she was carrying "suspicious material". The US said she was briefly detained - an accusation Iran rejected. It seems to be the first such incident since the IAEA began monitoring Iranian compliance with a 2015 nuclear deal. The US also warned that Iran might be "positioning itself to have the option of a rapid nuclear break-out" - the time it would take to acquire enough fissile material for one bomb - after it suspended another commitment under the accord. On Thursday, uranium enrichment resumed at the underground Fordo facility. Enriched uranium can be used to make reactor fuel but also nuclear weapons. It is the fourth such step Iran, which has insisted its nuclear programme is entirely peaceful, has taken in response to the sanctions reinstated by US President Donald Trump when he abandoned the nuclear deal last year. Under the accord, Iran agreed to limit its sensitive nuclear activities and allow in international inspectors in return for the lifting of economic sanctions. Mr Trump wants to force Iran to negotiate a new agreement that would place indefinite curbs on its nuclear programme and also halt its development of ballistic missiles. But Iran has so far refused. The other parties to the deal - the UK, France, Germany, China and Russia - have tried to keep it alive. But the sanctions have caused Iran's oil exports to collapse and the value of its currency to plummet, and sent its inflation rate soaring.