By Sune Engel Rasmussen in Beirut, Laurence Norman in Brussels and Aresu Eqbali in Tehran 

Iran said it would begin enriching uranium beyond limits set in a 2015 nuclear deal, marking the second intentional violation in a week and putting the multiparty accord at risk of complete collapse.

Since President Trump pulled out of the pact last year, Tehran has warned it would step up nuclear activities in a bid to raise the pressure on Europe, China and Russia to provide relief from crippling U.S. economic sanctions and to raise costs to Washington of continuing its maximum-pressure campaign.

On Sunday, Iran said it would begin making technical changes at its nuclear facilities to start enriching uranium above the 3.67% allowed under the nuclear deal, surpassing the limit by Monday morning. That move would bring Iran a small step closer to having fissile material that could be used in an atomic weapon.

"Tomorrow early morning we will have passed over 3.67%," Behrouz Kamalvandi, spokesman for Iran's atomic agency, said in a press briefing in Tehran.

He didn't say to what level Iran would enrich uranium but said that it was to supply fuel to power plants, indicating Iran will initially only enrich at 5% purity.

"If the 3.67% becomes 3.68%, or 4% or 5%, it is not important. This has a political strategic significance," said Abbas Araghchi, Iran's deputy foreign minister and a key player in the nuclear negotiations.

Iranian officials said the latest decision wasn't meant to undermine the nuclear accord, but that it was up to the remaining parties to live up to their commitments.

A top adviser to Supreme Leader Ali Khamenei on Saturday had hinted that Iran would increase enrichment to 5%. Iran has produced up to 20% enriched uranium in the past. Weapons grade uranium is 90%.

The White House on Sunday referred to its statement from July 1, when Iran breached the nuclear deal's cap on its stockpile of low-enriched uranium, which said "we must restore the longstanding nonproliferation standard of no enrichment for Iran...the regime must end its nuclear ambitions."

Iran maintains that it isn't violating the nuclear agreement but merely invoking a clause in the deal that allows for one side to cease partly or in full its commitments if another party isn't fulfilling its commitments. European countries say Iran hasn't gone through the necessary process to take that step.

A spokeswoman for European Union foreign policy chief Federica Mogherini said on Sunday the EU was "extremely concerned" by Iran's announcement. In comments coordinated with France, Germany and Britain, the spokeswoman said the EU would discuss next steps with Russia and China, the other participants in the 2015 nuclear deal.

"We had called upon Iran not to take further measures that undermine the nuclear deal," said the spokeswoman, Maja Kocijancic. "We strongly urge Iran to stop and reverse all activities inconsistent with its commitments under the nuclear deal."

Iran's brinkmanship could present Mr. Trump, who has spoken out against U.S. involvement in Middle East conflicts, with difficult questions of war and peace as he prepares for the 2020 elections.

The U.S. on Friday summoned a special session of the board of the International Atomic Energy Agency to consider Iran's actions, which will take place on July 10. Washington may push for a formal censure of Tehran. The Vienna-based agency oversees Iran's obligations under the 2015 deal.

"The international community must hold Iran's regime accountable," the U.S. said in a statement on Friday.

A spokesman for the United Nations agency said inspectors will be checking to see if Iran goes over the 3.67% limit.

Some European diplomats had hoped that Iran would take months to move clearly in violation of the agreement. Still, with Iran only increasing its uranium stockpile and enrichment levels gradually, it is likely to strengthen voices in Europe saying there is still time for diplomacy.

European officials have said they may trigger a dispute procedure in the accord if Iran continues to breach its obligations. That would start a weekslong process that could end with international sanctions being reimposed on Tehran. The EU spokeswoman said the bloc, which chairs the body that is supposed to resolve conflicts over the deal, could call a meeting of this commission in the coming days.

Sunday's move marks the end of a 60-day deadline issued by Iran in May, when it said it would cease to abide by certain commitments if European nations didn't do more to help offset the economic impact of U.S. sanctions.

Mr. Araghchi said Foreign Minister Javad Zarif on Sunday sent a letter to Ms. Mogherini stating the latest step and issuing another 60-day deadline to take further action, but he didn't disclose what those measures would be.

French President Emmanuel Macron spoke with Iranian President Hassan Rouhani on Saturday and said he was deeply concerned about Tehran's weakening of the accord, and "the consequences that would necessarily follow," according to a statement.

Messrs. Macron and Rouhani also agreed to explore by July 15 the conditions for a resumption of talks between the remaining parties to the agreement, the statement said.

Asked about the prospect of the U.S. joining in the talks, Mr. Araghchi said, "If the U.S. wants to take part in talks, on the condition of lifting its sanctions, it is possible in our view."

Tightened American sanctions have reduced Iranian oil exports by about 90%, causing severe ripple effects across the economy and driving Tehran to adopt a more confrontational stance.

In addition to the nuclear activities, the U.S. has accused Iran of attacking oil tankers and Saudi oil facilities -- charges that Iran denies. In June, Iran shot down an American spy drone.

The steps Iran has taken so far don't significantly impact on Iran's breakout time -- how long it would take Tehran to produce enough fissile material for one nuclear bomb. The nuclear deal was crafted to ensure Iran's breakout time was at least a year.

Notably, Iran has steered clear of actions or threats to other key parts of the deal. Western diplomats say Iran has, for the most part, allowed timely inspections of nuclear-related facilities by the IAEA's monitors.

It has also not threatened to disregard limits on the number and type of centrifuges it deploys to enrich uranium, a critical component of maintaining the one-year breakout time.

Under the deal, Iran was required to remove and place in storage two-thirds of its close to 20,000 centrifuges, most of which were basic machines of a type used decades ago in Western countries. Iran also placed in storage around 1,000 somewhat more advanced machines.

Were Iran to reinstall all its centrifuges or add more advanced ones, it could significantly reduce the time needed to produce a nuclear weapon to below one year, a threshold Trump administration officials say they are closely monitoring.

Iranian officials on Sunday said increasing the number of centrifuges wasn't necessary for now.

"We don't need to increase centrifuges," Mr. Kamalvandi said. "Adding centrifuges is an option the country's senior officials can choose."

Israeli Prime Minister Benjamin Netanyahu called the Iranian move a "dangerous step" and asked European leaders to fulfill their commitments to apply sanctions on Iran should it break the nuclear deal.

Dov Lieber in Tel Aviv contributed to this article.

Write to Sune Engel Rasmussen at sune.rasmussen@wsj.com and Laurence Norman at laurence.norman@wsj.com

(END) Dow Jones Newswires

July 07, 2019 10:40 ET (14:40 GMT)

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