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(AFP) - President Hassan Rouhani said Tuesday that Iran would resume uranium enrichment at an underground plant south of Tehran in its latest step back from a troubled 2015 agreement with major powers.

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.

Under the terms of the agreement, Iran has retained more than 1,000 first-generation centrifuges at the Fordow plant, which have been running empty or remained idle since it took effect.

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

His announcement came a day after tensions flared anew on the 40th anniversary of the US embassy siege and hostage crisis, with thousands in Tehran taking to the streets and Washington imposing fresh sanctions.

Iran said the resumption of enrichment at Fordow would be carried out transparently and witnessed by inspectors from the United Nations nuclear watchdog, the International Atomic Energy Agency.

But the European Union voiced disquiet at the new step away from the 2015 deal.

"We are concerned by President Rouhani's announcement today to further reduce Iran's commitments," EU spokeswoman Maja Kocijancic told reporters.

France urged Iran to "go back on its decisions, which contradict the accord."

Russia too expressed worry, despite its good relations with Iran.

"We are monitoring the development of the situation with concern," President Vladimir Putin's spokesman Dmitry Peskov told reporters.

"We support the preservation of this deal."

At the same time, Peskov said Moscow understood Tehran's concerns over the "unprecedented and illegal sanctions" imposed by Washington.

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

- 'Committed to negotiations' -

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.

Rouhani stressed that Iran remained committed to efforts to save the 2015 agreement, despite its phased suspension of some of its commitments.

"The fourth phase, like the three previous ones, is reversible," he said.

"We are committed to all the behind-the-scenes negotiations we have with some countries for a solution.

"Over the next two months, we will negotiate more."

Rouhani said Iran wanted to return to a situation in which "we can easily sell our oil, we can easily use our money in banks."

If that were achieved, "we will completely go back to the previous situation."

The EU warned on Monday that its continued support for the deal depended "on full compliance by Iran".

"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 said.

But Ellie Geranmayeh, an Iran expert at the European Council on Foreign Relations, said the measures were designed to increase Iran's leverage in future talks and to pressure Western countries to provide Iran with economic relief.

"This is not attempt to inch towards nuclear weapon," she wrote on Twitter. "#IAEA continues to provide intrusive monitoring."

On July 1, Iran said it had increased its stockpile of enriched uranium to beyond a 300-kg maximum set by the deal.

A week later, it announced it had exceeded a 3.67-percent cap on the purity of its uranium stocks.

On September 7, it fired up advanced centrifuges to boost its enriched uranium stockpiles.

On Monday, Iran announced a more than tenfold increase in enriched uranium production as a result of the steps back from the nuclear deal it had already undertaken.

The country's enriched uranium production has reached five kilogrammes per day, said Ali Akbar Salehi, the head of Iran's Atomic Energy Organisation, compared with 450 grams per day two months ago.

© Agence France-Presse