Iran has started enriching uranium to a purity level of 60 per cent in response to what it says was an Israeli attack on a key nuclear facility, the country's chief negotiator Abbas Araghchi said on Tuesday.
He was speaking before talks in Vienna on the 2015 nuclear deal.
Tehran blamed Israel for a power outage at the Natanz underground nuclear enrichment site on Sunday, that damaged centrifuges used to enrich uranium.
Israel has not officially commented on the accusation.
Mr Araghchi said Iran announced its plans in a letter to the International Atomic Energy Agency.
In addition to replacing the centrifuges damaged in Sunday's attack, it will add 1,000 new advanced centrifuges, he said.
According to Mr Araghchi, these new centrifuges will have 50 per cent more capacity than the damaged ones.
Behrouz Kamalvandi, deputy chairman and spokesman of the Atomic Energy Organisation of Iran, said on Tuesday that "the preparation for execution of the 60 per cent uranium enrichment will begin as of tonight in Natanz nuclear facility".
Under the terms of the 2015 nuclear agreement, Iran agreed to limit enrichment to 3.67 per cent for 15 years and drastically cut the country's stockpiles.
According to experts, the drastic increase in uranium enrichment is a serious escalation by Iran. In the past, Iran has never exceeded 20 per cent enrichment.
Hardliners in Tehran called for an end to the Vienna negotiations after the Natanz attack and vowed to respond with security forces.
In 2018, then US president Donald Trump withdrew from the Iran nuclear deal and imposed sanctions on the country.
In response, Tehran began to breach the terms of the Joint Comprehensive Plan of Action and tried to pressure Europe, Russia and China to maintain the agreement.
While US President Joe Biden has said he is keen to see a return of the agreement and of diplomacy, US officials demand Iran return to compliance before they will remove sanctions.
Iran, however, insists sanctions must be lifted before returning to compliance.
" I hope that I can go back to Tehran with the list of sanctions that will be lifted," Mr Araghchi said in Vienna.
"Otherwise, I don’t believe we can continue like this."