The April 12 imagery shows five specialized railcars near the uranium enrichment facility and the radiochemistry laboratory at the Yongbyon nuclear site, according to the Center for Strategic and International Studies.
"In the past these specialized railcars appear to have been associated with the movement of radioactive material or reprocessing campaigns," it said in a report.
"The current activity, along with their configurations, does not rule out their possible involvement in such activity, either before or after a reprocessing campaign," it added.
Yongbyon is the heart of North Korea's nuclear weapons program.
During a summit with US President Donald Trump in Vietnam in February, North Korean leader Kim Jong-un offered to dismantle the complex in return for significant relief from international sanctions weighing on the regime.
The summit broke down as Trump declined the offer and called for the dismantlement of all of the North's weapons of mass destruction programs.
Pyongyang has conducted six nuclear weapons tests and multiple tests of ballistic missiles designed to deliver the bombs to destinations including the mainland United States. (Yonhap)