North Korea has halted the nuclear reactor at its main atomic complex, probably to extract plutonium that could be used for weapons by reprocessing spent fuel rods, a South Korean news report said on Thursday, citing a government source.

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

"South Korea and the U.S. 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.