The UN nuclear watchdog chief has reported a "strong" water outflow from a reactor's cooling system at North Korea's key nuclear complex in Yongbyon, hinting at the possible test-running of the facility suspected to be designed to produce more fissile material for nuclear bombs.

International Atomic Energy Agency Director General Rafael Grossi explained his agency's observation of the light water reactor at the nuclear complex, 100 kilometers north of Pyongyang, during the agency's Board of Governors meeting in Vienna on Wednesday.

"Increased levels of activity have been observed at and near the LWR," he said in the statement posted on the IAEA website. "Since mid-October 2023, a strong water outflow has been observed from the LWR cooling system. These observations are consistent with the commissioning of the LWR."

Grossi highlighted the continuous activities at the Yongbyon complex, reiterating that the "obvious" continuation of the North's nuclear program is a "clear" violation of UN Security Council (UNSC) resolutions.

"The 5MW(e) reactor was shut down for three to four weeks during September and early-October 2023, however there are now indications of operation," he said. "There are also indications of ongoing operation of the reported centrifuge enrichment facility and its annex."

The IAEA chief also stressed that the North's Punggye-ri nuclear test site remains "prepared to support a new nuclear test," which he described as a "cause for serious concern."

He called the North's nuclear activity "deeply regrettable."

"I call upon the DPRK to comply fully with its obligations under relevant UNSC resolutions, to cooperate promptly with the agency in the full and effective implementation of its NPT Safeguards Agreement and to resolve all outstanding issues, especially those that have arisen during the absence of Agency inspectors from the country," he said.

DPRK stands for North Korea's official name, the Democratic People's Republic of Korea. NPT is the acronym for the Nuclear Non-Proliferation Treaty.

"The agency continues to maintain its enhanced readiness to play its essential role in verifying the DPRK's nuclear program," Grossi added. (Yonhap)