TEHRAN, Iran — Iran warned European nations Monday that it would breach in 10 days the landmark 2015 nuclear agreement unless they take action to alleviate the pressure of tightening U.S. sanctions in the coming weeks.

The spokesman for Iran's atomic energy agency, Behrouz Kamalvandi, said the country would exceed a cap on stockpiles of low-grade uranium on June 27 and threatened to raise enrichment purity beyond a 3.67 percent limit meant to prevent Iran from making weapons-grade material.

"This is an important test for Europe. It's to their detriment that the U.S. is making decisions for them," he said in a televised address from Arak heavy water plant. "Meetings and summits won't suffice. Once they take actionable measures, we can return to our previous commitments."

The announcement raises pressure on European nations that have urged Tehran to stick with a deal even after it was abandoned by the U.S. but have struggled to come up with a vehicle that would allow the Iranians to keep trading. It will also stoke further friction with the U.S., which has accused the Islamic Republic of being behind a spate of attacks on oil tankers near the Strait of Hormuz shipping chokepoint. Iran denies any wrongdoing.

The U.K. said security officials were meeting Monday to discuss the situation and the British government would consider renewed sanctions if Iran violates the nuclear deal.

Tensions in the Gulf have spiked since the U.S. stopped granting waivers to buyers of Iranian oil in early May, tightening sanctions slapped on the Islamic Republic after President Donald Trump pulled the U.S. out of the accord a year ago. Trump says he wants to negotiate a new deal that would also restrict Iran's missile programs and support for regional proxies. Iran says it is not seeking to develop a nuclear weapon.

While China, Europe and Russia have stuck with the so-called Joint Comprehensive Plan of Action, an Iranian violation would make it harder for them to stand up to U.S. pressure.

After waiting a year for action, Iran threatened on May 8 to gradually withdraw from the accord unless the remaining parties throw it an economic lifeline within 60 days.

It said it would no longer comply with a 300-kilogram cap on the storage of enriched uranium and a 130-ton limit on stocks of heavy water after the U.S. revoked key waivers that had allowed the Iranians to send excess heavy water to Oman and ship out surplus enriched uranium in exchange for natural, or yellowcake, uranium.

The U.S. move left Iran with a stark choice: submit to pressure and stop all enrichment or abandon some of its obligations under the accord.

The country has since accelerated more than four-fold the rate at which it is enriching low-grade uranium and said it could hit its cap on heavy water storage in 2 months, though there were plenty of domestic uses for the material.

Kamalvandi said European parties to the deal still had time to save it but the deadline would not be extended. If Europe gives the Iranians a way to maintain access to international oil and financial markets, Iran has promised to resume complying fully.

European nations have been working with Iran on a vehicle that would allow it to trade but progress has been slow and the Trump administration is now considering fresh measures that would effectively block that initiative.

Monday's announcement was made on the heels of a visit to Tehran by the EU's Helga Schmid, who heads a joint commission that mediates disputes under the nuclear accord.

Developments in the Gulf will be discussed "at length during the day," said the EU's top foreign policy official Federica Mogherini. She convened a meeting of the bloc's 28 foreign and defense ministers Monday.

An Iranian violation will turn up the heat on diplomats and monitors at the International Atomic Energy Agency responsible for monitoring compliance. IAEA Director General Yukiya Amano confirmed Iran had raised its rate of production last week and warned the nuclear deal was coming under "increasing tensions."

Inspectors are on the ground daily in Iran and any violation will be reported to IAEA members, Amano said. Overstepping stockpile limits could result in an extraordinary meeting of the IAEA's board and the start a process that could lead to the re-imposition of broader United Nations sanctions.