This Jan. 15, 2011, file photo, shows a part of Arak heavy water nuclear facilities, near the central city of Arak, 150 miles southwest of the capital Tehran, Iran. Credit: Mehdi Marizad | Fars News Agency via AP

Iran’s capacity to enrich uranium — the heavy metal used for bombs and nuclear power — has grown 25-fold since it began violating the landmark deal with world powers to protest the return of U.S. sanctions.

International Atomic Energy Agency inspectors said Monday that Iran’s stockpile of low-enriched uranium swelled almost two-thirds during the last quarter to more than 372 kilograms, according to a restricted document seen by Bloomberg. Iran’s now enriching about 100 kilograms of the metal a month, compared with just 4 kilograms back when it was observing the 2015 agreement’s conditions in June, and purifying the metal to 4.5 percent

Monitors confirmed Iran had restarted enriching at its Fordow complex built into the side of a mountain. New advanced centrifuges being tested at the country’s main enrichment plant in Natanz mean its rate of production could expand significantly, according to a senior diplomat familiar with the work.

The report is the latest blow to the beleaguered Joint Comprehensive Plan of Action, the deal jettisoned in 2018 by U.S. President Donald Trump that capped Iran’s enrichment ability. After withholding retaliation for a year in the hope that European countries could provide economic relief from crushing American sanctions, Iran also began violating terms of the agreement in July after the aid never materialized.

About 630 kilograms of low-enriched uranium must be purified to 90 percent to yield the 15 kilograms to 22 kilograms of weapons-grade uranium needed by an expert bomb-maker to craft a weapon.

The agreement was designed to ensure Iran would need at least a year to accumulate sufficient material if it ever decided to break out of the accord.

“Full commitment to the agreement remains crucial for our security, even if it is increasingly difficult to preserve it,” European Union foreign policy chief Federica Mogherini said in a statement. “We will continue our efforts to have a full implementation.”