Activity at North Korea's main nuclear reactor and reprocessing plant has significantly increased, suggesting that Pyongyang is trying to produce new weapons-grade plutonium, a U.S. research institute said Tuesday.

Satellite imagery taken on Aug. 22 shows brisk vehicular traffic "not previously observed" in front of the five-megawatt nuclear reactor and the nearby reprocessing plant at the country's Yongbyon nuclear complex, the website 38 North said.

"Significant vehicular traffic involving both the 5 MWe Reactor and the Radiochemical Laboratory complex, including the spent fuel receiving building, may indicate preparations for unloading spent fuel rods from the reactor for the purpose of producing new plutonium," 38 North said.

"If this explanation proves to be true, it would represent an important step towards the further development of North Korea's nuclear weapons stockpile," it said.

Other possible reasons include major maintenance or renovation activities as part of an ongoing program to modernize and upgrade the Yongbyon facility. The North could also be replacing contaminated equipment from the 5 MWe Reactor and moving it to the reprocessing complex for storage or decontamination, it said.

The graphite-moderated reactor has been the source of weapons-grade plutonium for the communist nation. The small reactor is capable of producing spent fuel rods which, if reprocessed, could give the regime enough plutonium to make one bomb a year.

The reactor has provided Pyongyang with weapons-grade plutonium that the regime used in its three nuclear tests conducted in 2006, 2009 and 2013. In addition to its plutonium program, the North has also built a uranium enrichment facility that gives Pyongyang a second way of building nuclear bombs.

"While international attention has recently been focused on North Korea's Sohae Satellite Launching Station and the possibility of a fourth long-range rocket test in October on the 70th anniversary of the Workers' Party of Korea, developments at Yongbyon are cause for concern and should be monitored closely by the international community to determine their purpose," 38 North said. (Yonhap)