Iranian Foreign Minister Mohammad Javad Zarif says the country has not violated a 2015 deal that seeks to limit its stocks of low-enriched uranium.
Iran has announced it has amassed more low-enriched uranium than permitted under its 2015 deal with major powers, its first major step beyond the terms of the deal since the US pulled out of it more than a year ago.
Iranian Foreign Minister Mohammad Javad Zarif, who announced the move, said it nevertheless did not amount to a violation of the accord, arguing that Iran was exercising its right to respond to the US walkout.
European powers, who remain party to the accord and have tried to keep it in place, urged Iran not to take further steps that would violate it.
But they held off on declaring the agreement void or announcing sanctions of their own.
UN nuclear watchdog the International Atomic Energy Agency, which monitors Iran's nuclear program under the deal, confirmed in Vienna on Monday that Tehran had breached the limit.
"We have NOT violated the #JCPOA," Zarif wrote on Twitter, referring to the deal by its formal title, the Joint Comprehensive Plan of Action.
"Para 36 of the accord illustrates why. We triggered & exhausted para 36 after US withdrawal," he wrote, referring to a paragraph of the accord that contains the mechanism for countries to resolve disputes.
He said Iran had waited 60 weeks after notifying other signatories before acting.
"As soon as E3 abide by their obligations, we'll reverse," he said, referring to European powers Britain, Germany and France, which Iran has demanded guarantee it the access to world trade envisioned under the deal.
The move is a test of European diplomacy after French, British and German officials had promised a strong diplomatic response if Iran fundamentally breached the deal.
The Europeans, who opposed last year's decision by Trump to abandon the agreement signed under his predecessor Barack Obama, had pleaded with Iran to keep within its parameters.
UK Foreign Secretary Jeremy Hunt said Britain wants to preserve the pact "because we don't want Iran to have nuclear weapons".
"But if Iran breaks that deal then we are out of it as well," he said.
Iran has said it aims to keep the accord in place but cannot abide by its terms indefinitely, as long as sanctions imposed by Trump have deprived it of the benefits it was meant to receive in return for accepting curbs on its nuclear program.
UN Secretary General Antonio Guterres is concerned about Iran's moves, spokesman Stephane Dujarric said.
"Such action by the Islamic Republic of Iran would not help preserve the plan, nor secure the tangible economic benefits for the Iranian people. It is essential that this issue ... be addressed through the mechanism established by the JCPOA," she said.
Israeli Prime Minister Benjamin Netanyahu said the European countries should "stand behind their commitments" and impose "automatic sanctions" on Iran.
Zarif said Iran's next move would be to enrich uranium beyond the maximum 3.67 per cent fissile purity allowed under the deal, a threshold Tehran has previously said it would cross on July 7.