Iran has begun installing advanced IR-6 centrifuges in a cluster at an underground enrichment plant in line with a longstanding plan and now intends to add two more such clusters, or cascades, the U.N. nuclear watchdog told its member states on Wednesday.

The moves are described in a confidential International Atomic Energy Agency report sent to member states shortly before the IAEA’s 35-nation Board of Governors passed a resolution criticising Iran for failing to explain uranium traces found at undeclared sites. Iran had warned of retaliation.

The IAEA report was seen by media and confirmed by the agency.

Iran’s commercial-scale Fuel Enrichment Plant at Natanz is its biggest and was built underground, apparently to protect it from potential aerial bombardment.

Iran informed the IAEA in a letter received on June 6 of its intention to install two “new cascades” of IR-6 machines at the underground plant, the report said.

On 8 June 2022, the Agency also verified that installation of the two ‘new’ IR-6 cascades had yet to begin, it added.

Earlier on Wednesday, before the board vote, Iran said it would “shut down” two IAEA cameras at an unspecified enrichment plant.