"Today the countdown to pass the 300 kilogrammes reserve of enriched uranium has started and in 10 days time... we will pass this limit," Iran's atomic energy organisation spokesman Behrouz Kamalvandi said at a press conference broadcast live.

"This is based on the Articles 26 and 36 of the (nuclear deal), and will be reversed once other parties live up to their commitments," he added, speaking from the Arak nuclear plant south-west of Tehran.

On May 8, President Hassan Rouhani announced that Iran would stop observing restrictions on its stocks of enriched uranium and heavy water agreed under the 2015 nuclear deal.

He said the move was in retaliation for the unilateral US withdrawal from the accord a year earlier, which saw Washington impose tough economic sanctions on Tehran.

Iran has threatened to go even further by July 8 unless remaining partners to the deal -- Britain, China, France, Germany and Russia -- help it circumvent US sanctions and especially enable it to sell its oil.

Under the agreement, Iran pledged to reduce its nuclear capacities for several years and allow international inspectors inside the country to monitor its activities in return for relief from international sanctions.

The deal set a limit on the number of uranium-enriching centrifuges, and restricted its right to enrich uranium to no higher than 3.67 percent, well below weapons-grade levels of around 90 percent.

It also called on Iran to export enriched uranium and heavy water to ensure that the country's reserves would stay within the production ceiling set by the agreement, yet recent US restrictions have made such exports virtually impossible.