Satellite images taken late last week appear to show activity at North Korea’s main nuclear site, a possible negative sign for diplomatic negotiations between the U.S. and Kim Jong Un's regime.

Images taken of the Yongbyon nuclear site on Friday revealed five “specialized” rail cars near the Uranium Enrichment Plant and Isotope/Tritium Production Facility which suggested the transportation of radioactive material, Washington’s Center for Strategic and International Studies, a U.S. think tank, said in a report on Tuesday.

A view of what researchers of Beyond Parallel, a CSIS project, describe as a probably 20-foot shipping container near the uranium enrichment plant at the Yongbyon Nuclear Research Center in North Pyongan Province, North Korea, in this commercial satellite image taken April 12, 2019 and released April 16. 

A view of what researchers of Beyond Parallel, a CSIS project, describe as a probably 20-foot shipping container near the uranium enrichment plant at the Yongbyon Nuclear Research Center in North Pyongan Province, North Korea, in this commercial satellite image taken April 12, 2019 and released April 16.  (CSIS/Beyond Parallel/DigitalGlobe 2019 via REUTERS)

“In the past, these specialized railcars appear to have been associated with the movement of radioactive material or reprocessing campaigns.” the report said. “The current activity, along with their configurations, does not rule out their possible involvement in such activity, either before or after a reprocessing campaign.”

TRUMP RECIPROCATES AFTER NORTH KOREA’S KIM SAYS HE’S OPEN TO THIRD SUMMIT

Other images seemed to show a large construction crane and several vehicles throughout the nuclear site.

HARRY KAZIANIS: TRUMP SUMMIT WITH SOUTH KOREA PRESIDENT COULD BRING PROGRESS IN NORTH'S DENUCLEARIZATION

The report went on to note that there was no steam coming from any of the buildings — which would be a sign that production had begun — and that aside from the railcars' presence, there was “no activity of significance”.

A view of what researchers of Beyond Parallel, a CSIS project, describe as specialized rail cars at the Yongbyon Nuclear Research Center in North Pyongan Province, North Korea, in this commercial satellite image taken April 12, 2019 and released April 16, 2019.

A view of what researchers of Beyond Parallel, a CSIS project, describe as specialized rail cars at the Yongbyon Nuclear Research Center in North Pyongan Province, North Korea, in this commercial satellite image taken April 12, 2019 and released April 16, 2019. (CSIS/Beyond Parallel/DigitalGlobe 2019 via REUTERS)

President Trump made strides toward the denuclearization of the Korean Peninsula last June when he and Kim met in Singapore. A second summit in February proved less effective however when negotiations broke down over what the U.S. said was Pyongyang’s excessive demands for sanctions relief in return for limited disarmament measures.

CLICK HERE TO GET THE FOX NEWS APP

Kim said last week he would be open to a third summit but only if Washington were to come to mutually acceptable terms with North Korea.

A view of vehicles near what researchers of Beyond Parallel, a CSIS project, describe as being the Experimental Light Water Reactor at the Yongbyon Nuclear Research Center in North Pyongan Province.

A view of vehicles near what researchers of Beyond Parallel, a CSIS project, describe as being the Experimental Light Water Reactor at the Yongbyon Nuclear Research Center in North Pyongan Province. (CSIS/Beyond Parallel/DigitalGlobe 2019 via REUTERS)

Trump acknowledged on Twitter that he too would be willing to meet again, continuing to stress the importance of sanctions and a nuclear-free North Korea.

The Associated Press contributed to this report.