Iran's underground Fordow uranium enrichment plant "will soon be back to full operation", Iranian President Hassan Rouhani said, hours after Tehran said uranium gas had been injected into centrifuges on Wednesday.
"Iran's fourth step in reducing its commitments under the [2015 nuclear deal with world powers] by injecting gas into 1,044 centrifuges begins today," Mr Rouhani tweeted.
On Tuesday, Iran said it would start injecting uranium gas into centrifuges at Fordow.
The move will be a highly symbolic breach that will complicate European efforts to salvage Tehran's nuclear deal.
Under the 2015 agreement, Iran agreed to turn Fordow into a "nuclear, physics and technology centre", where 1,044 centrifuges are used for purposes other than enrichment, such as producing stable isotopes that have many civil uses.
Iran has gradually scaled back its commitments to the deal under which it limited its nuclear programme, after the US reneged on the agreement and reimposed sanctions.
The pact allows Iran to only spin the centrifuges at Fordow, which is inside a mountain near the city of Qom, without injecting gas.
Uranium gas injection could allow production of enriched uranium, which is banned at the site under the pact.
"Starting from Wednesday, gas will be injected into centrifuges at Fordow as part of part of our fourth step to reduce our nuclear commitments to the deal," Mr Rouhani said in a televised speech.
Kazem Gharibabadi, Iran's ambassador to the International Atomic Energy Agency, said Iran had told the agency about "the start of injecting UF6 into centrifuges at Fordow".
This means the centre will become an active nuclear site rather than a research plant as permitted under the pact.
"The IAEA was requested to send its inspectors to monitor the process," Mr Gharibabadi said. The agency monitors Tehran's compliance with the deal.
Iran's nuclear chief, Ali Salehi, said Iran would enrich uranium to 5 per cent at Fordow on Wednesday.
Tehran could also enrich uranium to the 20 per cent level if needed, Mr Salehi said, "but right now there is no need for that".
The deal capped the level of purity to which Iran can enrich uranium at 3.67 per cent, which is suitable for civilian power generation and far below the 90 per cent nuclear weapons grade.
Iran denies ever having aimed to develop a nuclear bomb.
UN nuclear inspectors reported in July that Iran had increased enrichment to 4.5 per cent purity as its first step to decrease its nuclear commitments.
These measures will further complicate the chances of saving the accord, which European powers, Russia and the EU have called on Iran to respect.
UN Secretary General Antonio Guterres called on all parties to fully adhere to their commitments under the pact.
Responding to the announcement, the US accused Iran on Tuesday of "nuclear extortion" and vowed no easing of pressure.
"Iran has no credible reason to expand its uranium enrichment programme, 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 spokesman said.
"We will continue to impose maximum pressure on the regime until it abandons its destabilising behaviour, including proliferation-sensitive work."
The department said it would await verification by the IAEA, the UN nuclear watchdog that increased inspections under the 2015 accord.
In the UK, Foreign secretary Dominic Raab said: "Iran's actions clearly contravene the deal and pose a risk to our national security.
"We want to find a way forward through constructive international dialogue but Iran needs to stand by the commitments it made and urgently return to full compliance.”
Iran said on Monday it had accelerated enrichment by doubling the number of advanced IR-6 centrifuges in operation.
It said it was working on "a prototype called the IR-9, which works 50 times faster than the IR-1 centrifuges".
Mr Rouhani gave another two-month deadline to Britain, France and Germany to salvage the deal by protecting Iran's economy from US sanctions reimposed in May.
"We can't unilaterally accept that we completely fulfil our commitments and they don't follow up on their commitments," he said.
Tehran said talks were possible if Washington lifted sanctions, specifically on Iran's banking and oil sectors, and returned to the deal.
"All these measures are reversible if other parties fulfil their commitments," Mr Rouhani said.
"We should be able to sell our oil and to transfer its money into the country."