The announcement by Iran, later verified by the United Nations' nuclear watchdog, that it had followed through with its threat to increase the stockpiles raises the pressure on the countries — Russia, China, Germany, France and Britain, along with the European Union — trying to preserve the pact following the unilateral withdrawal of the United States last year.
European signatories to the nuclear deal, known as the Joint Comprehensive Plan of Action, or JCPOA, have warned of consequences, but have not yet said what they might be. British Prime Minister Theresa May's spokesman, James Slack, said Iran's announcement was "extremely concerning."
"We will continue working with our JCPOA partners — in particularly with German and France — to keep the nuclear deal in place. This is in our shared security interests," Slack said. "We have been consistently clear that our commitment to the JCPOA depends on Iran complying in full with the terms of the deal and we urge them to reverse this step."
In a statement, a spokeswoman for EU foreign policy chief Federica Mogherini, one of the guarantors of the pact, called on Iran "to reverse this step and to refrain from further measures that undermine the nuclear deal."
"The EU remains fully committed to the agreement as long as Iran continues to fully implement its nuclear commitments," spokeswoman Maja Kocijancic said. Germany's Foreign Ministry added it was "very concerned" Iran had exceeded the stockpile limits and called on Tehran to reduce the levels "and not further undermine the nuclear agreement."
It said it would "carefully consider the next steps" with its JCPOA partners. Meantime, the clock is ticking toward Tehran's July 7 deadline, when it has threatened to begin enriching uranium closer to weapons-grade levels unless the JCPOA countries offer significant relief from American sanctions.
JCPOA member Russia downplayed the significance of Monday's announcement that the stockpile limit had been exceeded, calling on the Europeans in the JCPOA not to over-react and on Iran not to push further.
"Exceeding the 300-kilogram limit causes regret, but shouldn't be overdramatized, it must be seen as a natural result of the preceding events," said Russia's Deputy Foreign Minister Sergei Ryabkov, adding that Iran had faced "unprecedented and unthinkable" pressure from U.S. sanctions.
"We are calling on all European members of the Joint Comprehensive Plan of Action not to heat up the situation," he said. "We are urging our Iranian colleagues to be extremely responsible regarding their actions, particularly those that concern Iran's fulfillment of comprehensive guarantees with the IAEA and an additional protocol to the agreement."
President Donald Trump withdrew the United States from the accord last year and imposed new sanctions on Iran in hopes of forcing Tehran into negotiating a wider-ranging deal. Since then, Iran's economy has been suffering and the other countries involved have been struggling to keep the pact alive.
At the same time, Europeans have been calling for both the U.S. and Iran to deescalate the rising tensions in the Middle East. The U.S. has been worried about international shipping through the Strait of Hormuz since tankers were damaged in May and June in what Washington has blamed on limpet mines from Iran, although Tehran denies any involvement. Last week, Iran shot down a U.S. Navy surveillance drone, saying it violated its territory; Washington said it was in international airspace.
The JCPOA deal promises economic incentives in exchange in return for curbs on Iran's nuclear program, and while the country had been sticking to the limitations so far, its increasing frustration with those nations' inability to offset American sanctions led to the threats on stockpiles and the increase of enrichment purity.
European countries had been pressing for Iran to comply in full with the accord, though have not specified what the consequences would be of failing to do so. Iran's state-run IRNA news agency quoted Foreign Minister Mohammad Javad Zarif suggesting Tehran was still willing to bargain if Europeans were willing to provide more economic incentives to help offset American sanctions.
"If Europeans do what they have to do, our measures are reversible," Zarif said, IRNA reported. It seemed likely the Europeans would take a wait-and-see approach until the July 7 deadline, based on diplomats' previous comments that the purity of uranium Iran enriches was seen as a much more important limitation.
The announcement will give the U.S. added leverage, however, in pressing others to join Washington in leaving the JCPOA. Last week, U.S. Special Representative for Iran Brian Hook met top European diplomats in Paris and said he wants them to get tougher on Iran, instead of clinging to the nuclear deal.
Britain, France and Germany had been hoping that their establishment of INSTEX, a complex barter-type system to protect companies from American sanctions, would address some of Iran's concerns. But after announcing Friday that it was active, Iran's Deputy Foreign Minister Abbas Araghchi said in Vienna that while INSTEX represented a step forward, it was not enough.
"Europeans need to buy oil from Iran, or to consider credit lines for this mechanism," he said.
Vladimir Isachenkov in Moscow, Jill Lawless in London, Kiyoko Metzler in Vienna and Lorne Cook in Brussels contributed to this story.