The UN's nuclear watchdog confirmed that Iran has enriched uranium at a level higher than the limit set in a 2015 international pact.
Inspectors of the International Atomic Energy Agency (IAEA) said in a statement that "on 8 July verified that Iran is enriching uranium above 3.67% U-235."
The announcement came hours after Tehran said it had exceeded the agreed cap and reached 4.5% enrichment in response to the United States withdrawing from the deal.
Iran announced on 8 May that it no longer considered itself bound to keep to limits of stocks of heavy water and enriched uranium agreed as part of the 2015 Joint Comprehensive Plan of Action (JCPOA).
The move came a year after US President Donald Trump pulled out of the landmark accord between world powers and Tehran, which says it has lost patience with perceived inaction by the remaining European partners.
Tehran is now pushing those remaining signatories to keep their promise to help Iran work around biting sanctions reimposed by the US in the second half of last year.
The JCPOA permitted Iran to only enrich uranium to the level of 3.67%, sufficient for power generation but far below the more than 90% level required for a nuclear warhead.
Iranian officials have hinted they may go up to 5%, the level needed to produce fuel for Iran's only nuclear power station.
Earlier this month the IAEA confirmed that Iran had slightly exceeded the deal's 300-kilogramme limit on its stockpile of low-enriched uranium.
The IAEA has scheduled a special meeting on Iran's nuclear programme at its Vienna headquarters for Wednesday.