International Atomic Energy Agency (IAEA) headquarters is pictured in Vienna, Austria June 20, 2019. (Photo: REUTERS)

North Korea has for years used spent fuel from a 5-megawatt nuclear reactor at Yongbyon to produce plutonium for its nuclear arsenal but a telltale discharge of warm water from a larger light-water reactor suggests it is coming online, too, the International Atomic Energy Agency said.

"The discharge of warm water is indicative the reactor has reached criticality," IAEA chief Rafael Grossi said in a statement, meaning the nuclear chain reaction in the reactor is self-sustaining.

The IAEA has not had access to North Korea since Pyongyang expelled its inspectors in 2009. The agency now observes the country mainly using satellite imagery. Without access, the IAEA cannot confirm the reactor's operational status, Grossi said.