A US research group says North Korea may be operating its uranium enrichment facility at the Nyongbyon nuclear site.
The 38 North group on Wednesday released its analysis of satellite images of Nyongbyon.
In a photo taken on December 19th, there is no snow on the roof of a building that's believed to house centrifuges.
The group says this indicates the building is emitting heat, suggesting the uranium enrichment facility is in operation.
Previous analyses show repair work for a reactor cooling system at the site. The latest images confirm the work is ongoing.
North Korea said at the inter-Korean summit in September last year that it is ready to shut down the Nyongbyon nuclear site if the US takes reciprocal steps.
Observers say the North aims to maintain its nuclear development capability while keeping a close eye on the US.