Pyongyang has conducted several nuclear tests, including one in January. The latest developments at North Korea's Yongbyon complex come amid heightened tensions over global nuclear security.
A US research institute on Thursday said it observed "suspicious activity" at North Korea's Yongbyon Radiochemical Laboratory in recent satellite images, namely exhaust gases at the facility.
"Exhaust plumes have rarely been seen there and none have been observed on any examined imagery this past winter. The plumes suggest that the operators of the reprocessing facility are heating their buildings, perhaps indicating that some significant activity is being undertaken, or will be in the near future," said analysts at the US-Korea Institute at John Hopkins University in Baltimore.
"Whether that activity … means reprocessing additional plutonium is underway or will be in the near future remains unclear," the report said.