"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 told a new conference broadcast live on state television.

The move "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.

Tensions between Iran and the United States have escalated ever since, with Washington bolstering its military presence in the region and blacklisting Iran's Revolutionary Guards as a terrorist organisation.

The US has also blamed Iran for last week's attacks on two tankers in the Gulf of Oman, a charge Tehran has denied as "baseless".

Iran has threatened to go even further in scaling down nuclear commitments 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.

"The current situation is sensitive" and there is still time for the deal's partners to save this agreement, Rouhani told the French ambassador to Tehran Philippe Thiebaud on Monday.

"The collapse of the JCPOA is undoubtedly not in the interest of Iran... the region and the world," he added.

- 'Save the 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, yet recent US restrictions have made such exports virtually impossible.

According to Rouhani, the ultimatum he issued last month was intended to "save the (deal), not destroy it".

The three European parties to the accord created a trade mechanism meant to bypass US sanctions, but their attempt was dismissed by Iran's supreme leader Ayatollah Ali Khamenei as a "bitter joke".

If world powers do not step up to help Iran, the atomic energy organisation spokesman warned further steps could be taken.

"They range from going to 3.68 percent to any other percent according to the country's needs," said Kamalvandi.

Authorities are still debating whether to "redesign or revive" the Arak reactor, he added.

Uranium enriched to much higher levels than Iran's current stocks can be used as the fissile core of a nuclear weapon, while heavy water is a source of plutonium, which can be used as an alternative way to produce a warhead.

"#Iran not bluffing about 60-day deadline for ceasing compliance with parts of nuclear deal," Ellie Geranmayeh, senior fellow at European Council on Foreign Relation said in a tweet.

She downplayed the importance of uranium and heavy water stockpiles increasing and said more worrying for Europe is Iran exceeding the enrichment level allowed in the deal.

"I imagine Tehran will do this incrementally, testing Europeans at each stage."

Germany has acknowledged the economic benefits Tehran hoped for from the deal were now "more difficult to obtain", but has urged Iran to fully respect the "extraordinarily important" nuclear deal.

"A point to Europeans: if the first step took time to be done, other steps, especially increasing enrichment... need no more than a day or two," said Kamalvandi.