Israel urged European states to sanction Iran, while Russia voiced regret but said the move was a consequence of US pressure, which has pushed the deal towards collapse.

Britain called on Tehran "to avoid any further steps away" from the landmark deal, and the UN said Iran must stick to its commitments under the accord.

"Iran has crossed the 300-kilogramme limit based on its plan" announced in May, Foreign Minister Mohammad Javad Zarif told semi-official news agency ISNA.

But he also said the move could be reversed.

The United States withdrew from the nuclear deal last year and hit Iran's crucial oil exports and financial transactions as well as other sectors with biting sanctions.

Tehran, which has sought to pressure the remaining parties to save the deal, on May 8 announced it would no longer respect the limit set on its enriched uranium and heavy water stockpiles.

It also threatened to abandon more nuclear commitments unless the remaining partners -- Britain, China, France, Germany and Russia -- helped it circumvent sanctions, especially to sell its oil.

"The United States and its allies will never allow Iran to develop nuclear weapons," White House spokeswoman Stephanie Grisham said in a statement, warning that "maximum pressure on the Iranian regime will continue until its leaders alter their course of action."

"It was a mistake under the Iran nuclear deal to allow Iran to enrich uranium at any level," the statement said.

Zarif insisted Iran had done nothing wrong. "We have NOT violated the #JCPOA," he tweeted, referring to the deal.

He referred to a paragraph in the pact outlining mechanisms to resolve disputes if one side believes the other is not meeting its obligations.

"We triggered & exhausted para 36 after US withdrawal," tweeted Zarif.

He said Iran would "reverse" its decision "as soon as E3 abide by their obligations" -- referring to the European parties to the deal: Britain, France and Germany.

Zarif's American counterpart Mike Pompeo accused Iran of using its nuclear program "to extort the international community and threaten regional security."

"The Iranian regime, armed with nuclear weapons, would pose an even greater danger to the region and to the world," he said in a statement.

- 'One mustn't dramatise' -

The International Atomic Energy Agency confirmed Iran had exceeded the limit that the deal imposed on its stockpile of low-enriched uranium (LEU).

A diplomat in Vienna, where the UN's nuclear watchdog is based, told AFP that Iran had exceeded the 300 kilogramme limit by two kilogrammes.

Russia's deputy foreign minister, Sergei Ryabkov, said Iran's move was a cause for "regret" but also "a natural consequence of recent events" and a result of the "unprecedented pressure" from the US.

"One mustn't dramatise the situation," Ryabkov, whose country is a close ally of Tehran, said in comments reported by Russian news agencies.

Britain's Foreign Minister Jeremy Hunt said on Twitter that London was "deeply worried" and urged Iran to "come back to compliance" with the nuclear deal.

UN chief Antonio Guterres said it was "essential" that Iran stick to the deal.

Israeli Prime Minister Benjamin Netanyahu urged European countries to impose sanctions on his country's arch-foe Iran.

- 'Europe's efforts not enough' -

The European Union said Friday after a crisis meeting aimed at salvaging the deal that a special payment mechanism set up to help Iran skirt the sanctions, known as INSTEX, was finally "operational" and that the first transactions were being processed.

But "the Europeans' efforts were not enough, therefore Iran will go ahead with its announced measures", Zarif said.

INSTEX, which "is just the beginning" of their commitments, has not yet been fully implemented, he added.

The 2015 deal saw Iran commit to never acquiring an atomic bomb, accept drastic limits on its nuclear programme and submit to IAEA inspections in exchange for a partial lifting of crippling international sanctions.

But US President Donald Trump's unilateral withdrawal from the deal on May 8 last year -- and subsequent sanctions -- plunged Iran into recession.

Exactly a year after the US withdrew, President Hassan Rouhani said Iran would temporarily cease to limit its stocks of heavy water and low-enriched uranium.

Iran has also threatened to start enriching uranium above the agreed maximum purification level of 3.67 percent from July 7. That remains far short of the 90 percent purity required to build a weapon.

The latest tensions coincide with a buildup of US forces in the Gulf and a series of incidents including Iran's shooting down of a US drone it claimed had entered its airspace.