Iranian President Hassan Rouhani  Photo: AFP

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 Organization said.

The suspension of uranium enrichment at the long secret plant was one of the restrictions Iran had agreed to on its nuclear program in return for the lifting of UN sanctions.

Iran's announcement that it would resume enrichment at the Fordow plant from midnight (20:30 GMT Wednesday) had drawn a chorus of concern from the remaining parties to the troubled agreement.

The resumption of enrichment at Fordow is Iran's fourth move away from the deal.

Uranium enrichment is the sensitive process that produces fuel for nuclear power plants but also, in highly extended form, the fissile core for a warhead.

Iran is now enriching uranium to 4.5 percent, exceeding the 3.67 percent limit set by the 2015 deal but less than the 20 percent level it had previously operated to and far less than the 90 percent level required for a warhead. Iran has always denied any military dimension to its nuclear program.

Iran revealed on Thursday that it had withdrawn the credentials of one International Atomic Energy Agency inspector last week after she triggered an alarm at the gate to Iran's other enrichment plant at Natanz, raising suspicion she was carrying a "suspect product."