The suspension of all enrichment at the underground facility near the Shiite holy city of Qom was one of the restrictions on Iran's nuclear activities that it accepted in return for the lifting of international sanctions in the 2015 landmark accord.

Tehran first announced the resumption of enrichment at Fordow last November, the fourth phase of its push since May 2019 to progressively suspend commitments to the deal.

It was in retaliation to Washington's abandonment of the accord in May 2018 followed by its unilateral reimposition of sanctions.

"Currently 1,044 centrifuges are enriching at Fordow," Ali Akbar Salehi, the head of Iran's atomic agency, told the Iranian parliament's news agency ICANA.

"We were committed in the JCPOA that these 1,044 machines do not carry out enrichment, but it is being done per dropped commitments as much as needed and we will stockpile the enriched material, too," he added, referring to the accord's official name, the Joint Comprehensive Plan of Action.

Iran's other walk-back steps included exceeding the accord's restrictions on enriched uranium reserves and enrichment level, development of advanced centrifuges, and foregoing a limit on its number of centrifuges.

In a joint statement in November, Britain, France, Germany and the European Union said Iran's decision to restart activities at Fordow was "inconsistent" with the 2015 deal.

The parties to the accord have called on Iran to return to its commitments, but Tehran insists the steps can be reversed once its economic benefits from the deal are realised.

The United Nation's nuclear watchdog said on September 4 that Iran's stockpile of enriched uranium now stands at more than ten times the limit set down in the 2015 deal.

Tensions between Tehran and Washington have escalated since the US pulled out of the deal, and flared in January when a US drone strike killed top Iranian commander Qasem Soleimani in Baghdad.

They have spiralled in recent months following a US push to extend an arms embargo on Iran that starts to progressively expire in October as well as reimposing UN sanctions on the Islamic republic.