Iran followed through on its threat to breach a central limit of its nuclear deal with major powers by accumulating more enriched uranium than allowed under the accord, the International Atomic Energy Agency (IAEA) said on Monday.

"We can confirm that IAEA Director General Yukiya Amano has informed the board of governors that the agency verified on 1 July that Iran's total enriched uranium stockpile exceeded [the deal's limit]," a spokesperson for the UN agency said in a statement.

Earlier, Iran's ISNA news agency reported that Foreign Minister Mohammad Javad Zarif said Iran had exceeded the relevant limit of 300 kilograms of uranium hexafluoride (UF6) in defiance of a warning by European co-signatories to stick to the deal despite U.S. sanctions.

However, Foreign Ministry spokesperson Abbas Mousavi said Iran's steps to decrease its commitments to the nuclear deal were "reversible."

Zarif said Iran's move did not amount to a violation of the accord, arguing that Iran was exercising its right to respond to the U.S. decision to walk out on the deal last year.

Last Wednesday, the IAEA verified Iran had roughly 200 kilograms of low-enriched uranium, just below the deal's 202.8-kilogram limit, three diplomats who follow the agency's work told Reuters. A quantity of 300 kilograms of UF6 (uranium hexafluoride) corresponds to 202.8 kilograms of LEU, which the IAEA describes as the basic ingredient used to make nuclear fuel.

An IAEA report sent to member states and obtained by Reuters put Iran's stock of LEU at 205 kilograms, above the deal's limit of 202.8 kilograms.

Enriching uranium to a low level of 3.6 per cent fissile material is the first step in a process that could eventually allow Iran to amass enough highly enriched uranium to build a nuclear warhead.

'Reverse this step,' British PM tells Iran

After talks on Friday in Vienna, Iran said European countries had offered too little in the way of trade assistance to persuade it to back off from its plan to breach the limit, a riposte to U.S. President Donald Trump's decision last year to quit the deal and reimpose economic sanctions.

Mousavi urged them on Monday to step up their efforts.

"Time is running out for them to save the deal," state TV quoted him as saying.

The deal between Iran and six world powers lifted most international sanctions against Iran in return for restrictions on its nuclear work aimed at extending the time Iran would need to produce a nuclear bomb, if it chose to, from roughly two to three months to a year.

Iran's decision on Monday drew the ire of U.S. President Donald Trump, who warned that Tehran is "playing with fire."

When asked if he had a message for Iran, he said "no," but added, "They know what they're doing."

U.S. President Donald Trump warned on Monday that Iran is 'playing with fire,' after the country breached its nuclear deal with world powers. (Saul Loeb/AFP/Getty Images)

The White House said it would continue to apply "maximum pressure" on Iran "until its leaders alter their course of action." It also said Iran should be held to a standard barring all uranium enrichment.

U.K. Prime Minister Theresa May's office said Britain was urgently considering its next moves along with its partners, and urged Iran to "reverse this step." Her foreign secretary, Jeremy Hunt, said he was "deeply worried" by Iran's announcement.

"We don't want Iran to have nuclear weapons," said Hunt, explaining that Britain wants to preserve the nuclear pact. "But if Iran breaks that deal then we are out of it as well."

UN Secretary General Antonio Guterres also expressed concern about Iran's actions, said his spokesperson Stephane Dujarric.

A European diplomat told Reuters there was a mechanism under the agreement to deal with "any inconsistencies," and it would be up to a joint commission of signatories to decide next steps.

'Never threaten an Iranian,' foreign minister says

Iran says its nuclear program is solely for peaceful purposes, including generating power. Its regional adversary, Israel, which Iran does not recognize, says the program presents it with an existential threat.

Joseph Cohen, head of the Israeli intelligence agency Mossad, urged the international community to stop Iran from "stepping up enrichment."

"Just imagine what will happen if the material stockpiled by the Iranians becomes fissionable, at military enrichment grade, and then an actual bomb," he told the Herzliya security conference before Zarif's announcement.

"The Middle East, and then the entire world, will be a different place. Therefore, the world must not allow this to happen."

In May, Washington piled pressure on Tehran by ordering all countries to halt imports of Iranian oil, and tensions have been growing in the Gulf ever since.

Washington has dispatched extra forces to the Middle East, and U.S. fighter jets came within minutes of conducting air strikes on Iran last month after Tehran downed an unmanned American drone.

In a speech on Monday broadcast on state TV, Iranian Zarif said: "Iran will never yield to pressure from the United States ... If they want to talk to Iran, they should show respect.

"Never threaten an Iranian ... Iran has always resisted pressure, and has responded with respect when respected."

Trump has called for negotiations with Iran with "no preconditions," but Tehran has ruled out talks until the United States returns to the nuclear pact and drops its sanctions.