Britain, reacting to the announcement, said it would “look at all options” if Iran breached its obligations under the agreement. Other European leaders, meeting in Luxembourg, urged Iran to adhere to the deal.

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“As of today, Iran is still compliant. And we strongly hope, encourage, expect that Iran continues to comply with its commitments," said European Union foreign policy chief Federica Mogherini, adding that the bloc and other signatories would base their response on whether Iran was violating the agreement.

The pact curbed Iran’s nuclear activities in exchange for major sanctions relief but has come under threat since the United States abandoned the accord and reimposed a near-total embargo on the Iranian economy in the fall.

Some European diplomats saw that 10-day deadline less as a firm plan to violate enrichment limits and more as an urgent call to Europe from Iran to deliver fresh concessions.

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Kamalvandi said European nations needed to take “practical measures” to offset wide-ranging U.S. sanctions and salvage Iran’s participation in the landmark deal.

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One European diplomat, speaking on the condition of anonymity to discuss internal deliberations, said that if Iran does violate the deal, Europe probably would trigger the pact’s mechanisms for arbitration and impose new sanctions. Such a response, however, would probably not be coordinated with the United States, the diplomat said, since most Europeans blame the Trump administration for the recent spike in tensions.

The United States has embarked on a “maximum-pressure campaign” to isolate Iran and force it to halt ballistic missile tests and support for proxy militias in the region.

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The move by Iran on Monday, however, comes amid a spiraling escalation in the Persian Gulf region after attacks against commercial tankers near the Strait of Hormuz last week. The United States accused Iran of targeting the two ships in the Gulf of Oman — a charge that Iranian officials deny.

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Iran had earlier threatened to close the Strait of Hormuz — a strategic waterway through which a fifth of the world’s oil passes — should U.S. sanctions cripple its own energy exports.

European leaders have urged caution in assigning blame for last week’s tanker attacks, which targeted Norwegian- and Japanese-owned ships carrying petrochemicals through the strait.

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Japan and Germany have urged the United States to provide evidence beyond a grainy video released by the U.S. Central Command that appeared to show an Iranian patrol boat removing a device from one of the ships. U.S. officials said they believed the device was an unexploded mine.

German Foreign Minister Heiko Maas said that his country was comparing U.S. and British intelligence assessments with those of Germany and other European services.

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“We can almost be certain” of the U.S. and British assessments that Iran is responsible for the attack on the tankers, he told reporters, but “we need to be very careful.”

Europe would consider imposing sanctions or “broad action” on Iran connected to the attacks on the tankers if Tehran was responsible, said Latvian Foreign Minister Edgars Rinkevics.

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“But that’s if we really find that Iran has been behind that, and I think that is still an open issue,” he said. “We really need to establish hard facts. We have now a lot of statements.”

Iranian plans to increase uranium enrichment added further pressure on European leaders. In early May, Iran gave Europe 60 days to comply with a number of conditions to save the deal without U.S. backing.

But Iran’s demands — which reportedly include stepping up oil exports that have plummeted as a result of U.S. sanctions in recent months — are unlikely to be fully met by Europe. Helping to boost such exports would constitute a major escalation of tensions between Europe and the United States.

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This year, the European Union created a payment channel — INSTEX — which was supposed to facilitate trade with Iran. In creating the mechanism, the bloc risked a spat with the United States, where officials fear such efforts to bypass the U.S. dollar in international trade could weaken Washington’s influence to impose sanctions abroad in the long run.

But INSTEX has been mired in problems, and most European companies have withdrawn from Iran. The payment channel also is not expected to facilitate oil exports, which would leave a key Iranian demand unmet.

Under the 2015 nuclear deal, Iran is allowed to enrich uranium to 3.67 percent, enough for use as fuel in nuclear power plants but far short of the more than 90 percent needed to produce fissile material for a nuclear bomb.

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Kamalvandi said Monday, however, that Iran would pass the stockpile limit June 27 and that it needs to enrich uranium to 20 percent for use in an old, U.S.-supplied research reactor in Tehran that produces radioisotopes for medical and scientific purposes.

Birnbaum reported from Brussels.