North Korea appears to be continuing activities at its Yongbyon nuclear complex but no clear evidence has been detected to determine if spent fuel rods have been transferred from the storage facility to produce plutonium, a US think tank has said.

The monitoring website 38 North made the assessment Friday, based on recent satellite imagery, also noting that minor activities have been spotted at the site's uranium enrichment plant while construction of a new wing of a lab building continued.

"The coal-fired Thermal Plant, which supplies steam to the RCL, has remained in continuous operation since early March, although the smoke plume is somewhat diminished on the most recent satellite imagery," 38 North said. RCL stands for the Radiochemical Laboratory.

The think tank said the provided information is "insufficient to definitively determine whether a new reprocessing campaign is underway or whether the activity is simply related to the processing of radioactive waste from previous campaigns."

Still, the North's continued construction of a new wing of one of the laboratory buildings and expansion of other facilities demonstrate Pyongyang's "determination to continue their research and improve their capabilities at the Yongbyon complex," it said. (Yonhap)