Satellite images from last week show movement at North Korea's Yongbyon nuclear site, Washington's Center for Strategic and International Studies (CSIS) said.
The April 12 images show railcars near the uranium enrichment facility and radiochemistry laboratory, and may be associated with the transfer and reprocessing of radioactive material, CSIS added.
According to the report, "In the past these specialized railcars appear to have been associated with the movement of radioactive material or reprocessing campaigns. The current activity, along with their configurations, does not rule out their possible involvement in such activity, either before or after a reprocessing campaign."
According to Channel News Asia, The US State Department declined to comment on the report, but a source familiar with government assessments said US experts did not believe the photos showed any significant nuclear activity.
Jenny Town, a North Korea expert at Stimson Centre, said it the timing was "interesting."
"Because there wasn't an agreement with North Korea on Yongbyon, it would be interesting timing if they were to have started something so quickly after Hanoi," she said, adding that "it would underscore that it is an active facility that does increase North Korea's fissile material stocks to increase its arsenal."
Earlier this month, US President Donald Trump said his relations with North Korean leader Kim Jong Un were "very good" and that "a third Summit would be good in that we fully understand where we each stand."
In February, a summit between Trump and Kim was "productive" but ended with no deal signed.