"The process of increasing the 'capacity and production pace' of enriched uranium and heavy water has started since the day the president (Hassan Rouhani) ordered it," Behrouz Kamalvandi, spokesman for Iran's Atomic Energy Organisation, told the semi-official ISNA news agency.

On May 8, Rouhani announced that Iran would stop observing restrictions on stocks of enriched uranium and heavy water agreed under the 2015 nuclear deal with major world powers.

He said it was in retaliation for the unilateral US withdrawal last year from the agreement known as the Joint Comprehensive Plan of Action (JCPOA) and sanctions imposed by Washington on Tehran.

Since Rouhani's announcement, Iran has ceased to limit its stocks of heavy water and enriched uranium to 130 tonnes and 300 kilogrammes respectively as agreed under the nuclear deal.

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.

"Iran no longer sees itself committed to the ceiling," Kamalvandi told ISNA.

"If we cross this limit, it is no longer our problem but that of the other parties" to the deal which Iran reached with the Britain, China, France, Germany, Russia and the United States, he said.

One way to cross the limit is for Iran to stop selling its surplus aborad, made even more likely considering that recent US restrictions against Iran's enriched uranium sales make exports virtually impossible.

In his announcement last week Rouhani threatened to go further if the Europeans failed to start delivering on their promises to help Iran circumvent US sanctions within 60 days.