Iran has announced it will break a limit set on uranium enrichment, in breach of the landmark 2015 deal designed to curb its nuclear ambitions.
Deputy foreign minister Abbas Araqchi said Iran still wanted to salvage the deal but blamed European countries for failing to live up to their own commitments.
The US unilaterally withdrew from the agreement in 2018.
It has since reimposed tight sanctions affecting the Iranian economy.
The Iranian announcement marks the latest breach of the accord.
In May, Iran announced it would step up its production of enriched uranium, which can be used to make fuel for reactors but also for nuclear weapons.
The country has already stockpiled more enriched uranium than permitted under the terms of the deal.
Iran has strongly denied that it has any intention of building nuclear weapons.
What has Iran announced?
On the anniversary of the US withdrawal, Iran gave a 60-day deadline to the remaining signatories of the deal - China, France, Germany, Russia and the UK - to protect it from US sanctions.
Speaking at a press conference on Sunday at the end of that deadline, Mr Araqchi said Iran would start enriching uranium above a concentration of 3.67% within a few hours.
Officials previously said this would mean a concentration of about 5%. Iran's nuclear power plant in Bushehr only requires "low-enriched" fuel of about this level.
Weapons-grade uranium is 90% enriched or more.
The ramped-up enrichment programme may be designed to exert pressure on the diplomatic negotiations, as Mr Araqchi said Iran would keep reducing its commitment to the 2015 deal every 60 days.
Another nuclear reactor, used for research purposes in Tehran, does require higher-enriched fuel, at about 20% - the highest level Iran had been producing prior to the nuclear deal.
But Iran's Atomic Energy Organisation spokesman Behrouz Kamalvandi said the country already had a sufficient stockpile for that purpose.
"We will enrich uranium based on our needs," he said. "Right now we don't need to enrich uranium for the Tehran reactor."
Mr Araqchi stressed that diplomacy was still an option, provided the sanctions are lifted.
A spokesman for the UN's nuclear watchdog, the International Atomic Energy Agency (IAEA), said their inspectors would report back to headquarters "as soon as they verify the announced development".-BBC