The International Atomic Energy Agency says North Korea may have started operations at a new nuclear reactor. It expressed concern that the move could fuel the North's weapons program.

IAEA head Rafael Grossi said his team has seen "increased levels of activity," including warm-water discharges, around the Light Water Reactor at the Nyongbyon nuclear complex, which indicates the facility has reached criticality.

Grossi said he cannot confirm the findings because inspectors have no direct access to the site after being barred by North Korean authorities. Still, he said the facility could be used to produce plutonium for weapons. He added that further development of the North's nuclear program would be "deeply regrettable."

North Korean leader Kim Jong Un knows that starting up a new reactor would violate UN resolutions. However, he continues to be defiant. He warned earlier this week that his country would not hesitate to launch a retaliatory nuclear attack.