The United States has expressed serious concern over North Korea showing signs of starting the operation of a new experimental light-water reactor in its Yongbyon nuclear complex.

The U.S. Mission to the United Nations in Vienna expressed concern in response to an inquiry by Yonhap News regarding a recent statement by the head of the International Atomic Energy Agency (IAEA).

The mission said that without the IAEA's monitoring and assistance, significant risks extend to North Korea, the region, and the global nuclear industry, adding that the North's illegal nuclear and ballistic missile programs pose a serious threat to global peace and security.

IAEA Director General Rafael Grossi told the agency's board of governors on Thursday that the reactor appears to have gone into operation for the first time, citing a strong outflow of water observed from the reactor's cooling system since mid-October.

The IAEA chief said that such observation is “consistent with the commissioning of the reactor,” adding that the “discharge of warm water is indicative of the reactor reaching criticality.”

Grossi issued a similar assessment in November as well, saying that North Korea appears to be test-running the reactor, citing a strong outflow of water from the reactor's cooling system.