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The United States accused Iran on Tuesday of "nuclear extortion" and vowed no let-up in pressure after the clerical regime said it would resume uranium enrichment at the key Fordow plant.

"Iran has no credible reason to expand its uranium enrichment program, at the Fordow facility or elsewhere, other than a clear attempt at nuclear extortion that will only deepen its political and economic isolation," a State Department spokesperson said.

"We will continue to impose maximum pressure on the regime until it abandons its destabilising behaviour, including proliferation-sensitive work."

President Hassan Rouhani said Iran would resume enrichment at the plant near the Shia holy city of Qom that was suspended under a 2015 nuclear accord with the United States and five other powers.

It was the latest action by Iran to seek tangible benefits from the deal, from which the United States withdrew as President Donald Trump imposed sweeping sanctions aimed at reducing Tehran's regional role.

The State Department said it would await verification by the International Atomic Energy Agency, the UN nuclear watchdog which stepped up inspection under the 2015 accord.

The suspension of all enrichment at the Fordow plant 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.

Rouhani recalled that under the terms of the agreement Iran had retained more than 1,000 centrifuges at the plant which had been running empty since it went into effect.

"Starting from tomorrow (Wednesday), we will begin injecting (uranium hexafluoride) gas at Fordo," 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 UN nuclear watchdog, the International Atomic Energy Agency (IAEA).

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But Russia, which has close ties with Iran, expressed concern about the latest move.

"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 its commitments.

'Committed to negotiations'

Iran has repeatedly warned the remaining parties to the deal - the UK, 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 European Union warned Monday that its continued support for the deal depended on Tehran fulfilling its commitments.

Maja Kocijancic, spokeswoman for EU diplomatic chief Federica Mogherini, said the bloc "remains committed" to the deal but "our commitment... depends 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."

On 1 July, Iran said it had increased its stockpile of enriched uranium to beyond a 300-kilo maximum set by the deal, and a week later, it announced it had exceeded a 3.67-percent cap on the purity of its uranium stocks.

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

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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.

Enriched uranium production has reached five kilogrammes per day, Ali Akbar Salehi, the head of Iran's Atomic Energy Organisation, told reporters.

That compares with the level of 450 grams two months ago.

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