Iran resumed uranium enrichment at its underground Fordow plant south of Tehran on Thursday, in a new step back from its commitments under a landmark 2015 nuclear deal.

Engineers began feeding uranium hexafluoride gas into the plant’s mothballed enrichment centrifuges in “the first minutes of Thursday”, the Iranian Atomic Energy Organisation said.

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 that it would resume enrichment at the Fordow plant from midnight (8:30pm GMT Wednesday) 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 reimposed 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.

The same day, Iran also announced it had cancelled the accreditation of a UN nuclear inspector after she triggered an alarm last week at the entrance to the Natanz uranium enrichment plant.

The check at the entrance gate to the plant in central Iran had “triggered an alarm” raising concern that she could be carrying a “suspect product” on her, the Iranian Atomic Energy Organisation said in a statement posted online.

As a result, she was denied entry, it added, without specifying whether or not anything had been found in her possession.

The Iranian organisation said it had reported the incident to International Atomic Energy Agency (IAEA) and notified it that its inspector’s accreditation had been withdrawn.

She had since left Iran for Vienna, where the IAEA is based, it said, without saying when.

“Iran’s representative to the IAEA will present a full report on the matter” in Vienna later Thursday, the Iranian organisation added.

According to a source close to the IAEA, the 35 members of its council of governors will hold a special meeting dedicated to Iran.

Under a landmark 2015 deal between Iran and major powers, its nuclear facilities are subject to continuous monitoring by the IAEA.