International Atomic Energy Agency inspectors last week detected uranium enriched to 84% purity in Iran, just below the 90% level needed for nuclear weapons, Bloomberg reported on Sunday.

The report, citing two senior diplomats, said the U.N.’s nuclear watchdog was trying to clarify how Tehran accumulated the material, which is at the highest level of purity found by monitors in the country to date.

Iran previously told the IAEA that its centrifuges were configured to enrich uranium to 60% purity.

Earlier this month, the IAEA chastised the Islamic Republic for modifying the connection between the two groups of high-tech machines at its Fordow plant that enrich uranium to up to 60% purity.

The modification was discovered during an unannounced inspection on Jan. 21 at the Fordow Fuel Enrichment Plant (FFEP), a location built into a mountain where inspectors are beefing up checks after Iran said it would drastically increase enrichment.

In a confidential report to member states obtained by Reuters, the IAEA stated that “they were interconnected in a way that was substantially different from the mode of operation declared by Iran.”

It was unclear exactly where the 84% enriched uranium was found.

The IAEA is currently preparing its quarterly Iran safeguards report ahead of a March 6 Board of Governors meeting in Vienna.

IAEA Director General Rafael Mariano Grossi last month called the moribund 2015 nuclear deal between Iran and world powers an “empty shell,” and said Tehran has sufficient nuclear material for several atomic bombs if it is enriched to weapons-grade levels.