A US think tank says it has confirmed signs of ongoing activity at a nuclear facility in North Korea.

The Center for Strategic and International Studies announced the results of analyses of satellite images taken on Monday and Tuesday of the Nyongbyon nuclear research facility.

The images showed three specialized railcars, each 10 to 13 meters long, on a railway track near a nuclear fuel production facility.

The center says the cars were not observed in imagery from Sunday.

The railcars have been associated with the movement of radioactive material in the past. They were last seen in November last year.

Analysts at the center point to the possibility of North Korea using the railcars for the outbound shipment of liquid or solid radioactive waste.

They say they do not know whether this is linked to the new strategic weapon the North has referred to, but it does prove continued activity at the site.

nhk