Iran manufactured a record volume of uranium enriched just below the levels needed for nuclear weapons, complicating efforts to negotiate a peaceful resolution to international concerns over the Islamic Republic’s atomic ambitions.

In its first report since U.S. President Donald Trump’s administration began negotiations with Tehran, the International Atomic Energy Agency said its accumulation of highly enriched uranium rose by almost 50% to 409 kilograms (902 pounds) over the last three months. That volume of material could quickly be enriched to form the core of about 10 nuclear bombs, should Iran choose to pursue weapons.

“The rapid accumulation of highly-enriched uranium is of serious concern,” IAEA Director General Rafael Mariano Grossi wrote in the 22-page report seen by Bloomberg. “The agency will not be in a position to provide assurance that Iran’s nuclear program is exclusively peaceful.”

The report indicates significant hurdles remain before Iran can assuage the concerns of international monitors who currently vet the country’s nuclear program. Even as Tehran’s diplomats seek a nuclear detente with the U.S., Iranian officials continue to stonewall an IAEA probe into uranium particles detected at undeclared locations.

“The agency concludes that Iran did not declare nuclear material and nuclear-related activities at three undeclared locations,” Grossi wrote in the report, which could be used to refer Tehran to the U.N. Security Council.

Iranian and U.S. officials held their latest talks over Tehran’s atomic activities in Rome earlier this month. Both sides said some progress had been made and Trump described the negotiations as “very good.”

Iran addressed concerns over its enrichment ahead of the IAEA report’s publication. While the Islamic Republic continues to reject pursuit of a weapon, it won’t forgo uranium enrichment, Foreign Minister Abbas Araghchi said in remarks broadcast on state television.

“Them telling us ‘you shouldn’t enrich because we’re worried’ is just not acceptable to the Iranian people,” Araghchi said. “It’s basically accepting someone else’s dominance.”

The IAEA’s board of governors convenes June 9 in Vienna to discuss the latest findings. European countries have left open the possibility of referring Iran back to the U.N. Security Council, where broad international sanctions could be reimposed.

While IAEA monitors — who continue on-the-ground verification activities in Iran — have “no credible indications” the country is hiding weapons work, they’ve grown more worried by recent announcements.

“Repeated statements by former high-level officials in Iran related to Iran having all capabilities to manufacture nuclear weapons continue to provide concerns,” the agency wrote.

While Iran also continues to produce uranium enriched to lower levels, international concern has focused on its growing 60% stockpile. That material could quickly be upgraded to levels typically used in nuclear weapons — a concern long held by the U.S., Europe and Israel. Iran has always denied its nuclear development is for military use, saying the focus is on energy and civilian technology.

After pulling out of an international deal that limited Iran’s nuclear activity in exchange for sanctions relief during his first term, Trump has oscillated between support for negotiations and reviving a so-called maximum pressure campaign to weaken the Persian Gulf nation’s economy. Oil prices temporarily spiked last week after reports suggested Israel was preparing to bomb Iran’s nuclear sites, potentially dragging the U.S. into another Middle Eastern conflict.

The IAEA’s report “presents a stark picture that serves as a clear warning sign,” Israeli Prime Minister Benjamin Netanyahu’s office said Saturday in a statement, adding that the scope of Iran’s enrichment activities is “alarming.”

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(With assistance from Arsalan Shahla and Fadwa Hodali.)

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