"In the coming hours, the process of injecting (uranium hexafluoride) gas into the centrifuges at the Fordow site will be finalised in the presence of inspectors" from the International Atomic Energy Agency, said Behrouz Kamalvandi, quoted by the semi-official news agency ISNA.

The production of enriched uranium will be "operational from midnight", he added.

Iran announced on Tuesday that it would resume uranium enrichment at the underground plant south of Tehran in its latest step back from a troubled 2015 agreement with major powers.

The suspension of enrichment at the Fordow plant was one of the main curbs on its nuclear activities that Iran accepted four years ago in return for the lifting of international sanctions.

But Washington's unilateral abandonment of the nuclear deal in May last year followed by its reimposition of crippling sanctions prompted Iran to begin a phased suspension of its own commitments.

The move comes the day after the deadline set by Tehran for the remaining parties to the deal -- Britain, China, France, Germany and Russia -- to come up with a mechanism that would allow foreign firms to continue to do business with Iran without incurring US penalties.

The move is the fourth step away from nuclear commitments by Iran in response to Washington's abandonment of the agreement.