The operation of the 5 megawatt nuclear reactor at the Yongbyon nuclear complex has been suspended since late September, according to intelligence assessment by US and South Korean authorities, the report said.

"South Korea and the US believe this could be a sign of reprocessing work being done to obtain weapons-grade plutonium," the Donga Ilbo newspaper quoted a government source as saying.

Reprocessing of spent fuel rods removed from a nuclear reactor is a step taken before plutonium is extracted. The Yongbyon nuclear complex is the North's main source of plutonium that it likely has used to build nuclear weapons.