Iran has resumed uranium enrichment at its underground Fordow plant south of Tehran in a new step back from its commitments under the 2015 nuclear deal. The Iranian authorities also say they have cancelled the accreditation of a UN nuclear inspector.
Engineers restarted the plant's mothballed enrichment centrifuges in "the first minutes of Thursday", the statement said.
Tehran separately announce that it was cancelling the accreditation of one UN nuclear inspector.
The suspension of uranium enrichment at the long-secret plant was one of the restrictions Iran had agreed to on its nuclear programme in return for the lifting of UN sanctions.
Iran's announcement on Wednesday that it would resume enrichment at the Fordow plant from midnight had drawn a chorus of concern from the remaining parties to the troubled agreement.
Britain, China, France, Germany and Russia have been trying to salvage the hard-won deal since Washington abandoned it in May last year and re-imposed crippling unilateral sanctions.
They say Iran's phased suspension of its obligations under the deal since May makes that more difficult.
The resumption of enrichment at Fordow is Iran's fourth move away from the deal.
Uranium enrichment is the process that produces fuel for nuclear power plants but could also, in highly extended form, produce the fissile material essential for a warhead.
Iran has always denied any military dimension to its nuclear programme.