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, Ednews reports citing Reuters.

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.

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.

North Korea has also operated uranium enrichment facilities, which is a separate source of material that could be used for nuclear weapons.

"The possibility of a nuclear test by North Korea is not ruled out," Donga Ilbo quoted a senior government official as saying, without elaborating on what analysis pointed to the assessment the move may be related to a nuclear test.

South Korean Defense ministry spokesman Jeon Ha-gyu declined to comment on the details of the report but said U.S. and South Korean intelligence authorities are closely monitoring related developments.