The U.S.-based North Korea monitoring Web site "38 North" says there have been at least two unreported reprocessing campaigns in North Korea since last fall to produce plutonium.

"38 North" reported on Friday that the finding is based on thermal satellite imagery analysis of the Yongbyon Nuclear Scientific Research Center from last September to this past June. 

The North's Radiochemical Laboratory was said to have operated intermittently and there appeared to have been at least two unreported reprocessing campaigns to produce an undetermined amount of plutonium that can further increase North Korea’s nuclear weapons stockpile. 

"38 North" also cited increased thermal activity at the Uranium Enrichment Facility but said it was unclear if it was the result of centrifuge operations or maintenance operations.

It said centrifuge operations would increase the North’s enriched uranium inventory but based on imagery alone, it is not possible to conclude whether the plant is producing low or highly enriched uranium.

[Photo : ]