The Vienna-based nuclear watchdog said in a report the findings pointed to a steady expansion of North Korea's nuclear infrastructure at multiple locations.
In the report, dated August 28, the IAEA said it relied on satellite imagery and open-source material, including photographs published by North Korean state media, to track Pyongyang's nuclear activities. The agency has had no inspectors in North Korea since 2009.
It identified a new two-storey building at Yongbyon as the site's second uranium enrichment facility by comparing state media photographs of a June visit by North Korean leader Kim Jong-un with satellite images taken during the building's construction.
Photographs from the visit indicate gas centrifuge cascades are installed on both the upper and lower floors of the main building, the report said.
It added the building could accommodate up to 28 of the cascades shown in the photographs.
The report also said there were indications that operations began in January at a uranium enrichment facility in Kangson, near the capital Pyongyang, within an annex that had been constructed in 2024.
The IAEA said the developments in North Korea's nuclear program violated UN Security Council resolutions.
"The ongoing operation of enrichment facilities at Kangson and Yongbyon and the reported further expansion of the DPRK's production of weapons-grade nuclear material is a cause for serious concern," IAEA Director General Rafael Grossi said in a statement to the board of governors, referring to the North's official name, the Democratic People's Republic of Korea.
The report accompanied Grossi's statement.
Pyongyang has repeatedly rejected the agency's authority.
North Korea's mission to the United Nations in Vienna said last September that North Korea's status as a nuclear weapons state had become irreversible and the IAEA had "no legal right … to interfere in the internal affairs of a nuclear weapons state which exists outside the NPT", referring to the Non-Proliferation Treaty.
