A U.S. think tank said on Thursday that some facilities at North Korea's main nuclear complex in Yongbyon appear to have resumed operations. 

The Washington-based Center for Strategic and International Studies(CSIS) presented the assessment in a report citing thermal infrared satellite imagery of the Yongbyon Nuclear Research Center. 

In the report released through the CSIS' website Beyond Parallel, the imagery provides "strong indications" of activities involving the heating of buildings and resumed operations of facilities in March and April. 

The report said that these indications suggest that the Radiochemistry Laboratory, its associated thermal plant and the centrifuge plant resumed operations during the early part of March and have continued into mid-April.

It added that this renewed activity is likely an indicator of a new reprocessing campaign designed to expand North Korea's inventory of fissile material for nuclear weapons. 

The report said the activity could also be a strategic move by North Korean leader Kim Jong-un to continue "slowly ratcheting up pressure" on both South Korea and the United States.

[Photo : YONHAP News]