Iranian Foreign Minister Mohammad Javad Zarif said that the country increased the level of its low-enriched uranium production as it had announced earlier "in line with Articles 26 and 36 of the 2015 nuclear deal", officially known as the Joint Comprehensive Plan of Action (JCPOA), Press TV reported.
The move is thought to be Tehran's first major breach of the accord since US President Donald Trump withdrew from the agreement in 2018 and reimposed sanctions on Tehran.
The deal, which Iran had signed with the P5+1 group of countries the US, Britain, France, Russia and China plus Germany in July 2015 had limited its uranium enrichment in exchange for an easing of international sanctions.
"According to my information, Iran has surpassed the 300 kg limit (in producing low-enriched uranium) and we had already announced (that we were planning to do) this," Zarif, one of the chief architects of the deal, told ISNA news agency.
Calling Tehran's latest moves "reversible", Foreign Ministry spokesperson Abbas Mousavi said: "We told the Europeans that if more practical, mature and complete measures were taken, Iran's reduction (to its) commitments could be reversed. Otherwise, we will continue."
Tehran had threatened to surpass the maximum permitted amount of enriched uranium in retaliation to crippling US economic sanctions.
During talks in Vienna on Friday, European countries still party to the deal made a last-ditch effort to persuade Iran to back off from plans to breach the limit.
Earlier, Fars news agency cited a source as saying that International Atomic Energy Agency (IAEA) inspectors had measured the stockpile on Monday and confirmed that it had surpassed the cap.
An IAEA spokesman told the BBC that it was aware of the reports, adding: "Our inspectors are on the ground and they will report to headquarters as soon as the LEU (low-enriched uranium) stockpile has been verified."
Iran said in May that it had quadrupled its production of low enriched uranium. The announcement increased tensions in the region and set off a series of provocative moves by Washington and Tehran.
( With inputs from IANS )