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.

“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.”

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

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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”.

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.

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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.

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.