The suspension of all enrichment at the Fordow plant in the mountains near the Shiite holy city of Qom was one of the restrictions on its nuclear activities that Iran accepted in return for the lifting of international sanctions.

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

Rouhani recalled that under the terms of the agreement Iran had retained more than 1,000 centrifuges at the plant which had been running empty since it went into effect.

"Starting from tomorrow (Wednesday), we will begin injecting (uranium hexafluoride) gas at Fordo," Rouhani said in a speech broadcast by state television.

Iran said the whole process would be carried out transparently witnessed by inspectors from the UN nuclear watchdog, the International Atomic Energy Agency.

The move is the fourth announced by Iran since it began responding to Washington's abandonment of its commitments.

Iran has repeatedly warned the remaining parties to the deal -- Britain, China, France, Germany and Russia -- that the agreement can only be rescued if they help it circumvent US sanctions.

European governments have strived to come up with a mechanism that would allow foreign firms to continue to do business with Iran without incurring US penalties.

But to Iran's mounting frustration, their efforts have so far failed to have any significant impact.

The European Union warned Monday that its continued support for the deal depended on Tehran fulfilling its commitments.

Maja Kocijancic, spokeswoman for EU diplomatic chief Federica Mogherini, said the bloc "remains committed" to the nuclear deal.

"We have continued to urge Iran to reverse such steps without delay and to refrain from other measures that would undermine the nuclear deal," Kocijancic told reporters in Brussels.

"But we have also been consistent in saying that our commitment to the nuclear deal depends on full compliance by Iran."