North Korea may have restarted one of its nuclear reactors, satellite pictures taken earlier this year suggest.
According to the Institute for Science and International Security, images of the Yongbyon plant show patterns of melting snow indicating new activity.
Yongbyon’s reactor was shut down in 2007 but was restarted in 2013.
The think-tank’s report says that its assessment in late 2014 was that the reactor at Yongbyon, in the country’s west, had been “shut down or partially shut down” but more recent images suggested the plant “may be operating at low power or intermittently”.
As well as the patterns of melting snow, the satellite photos are also said to capture a stream of warm water coming out of the reactor’s discharge pipeline and steam rising off the turbine.
In 2008, North Korea walked away from six-party talks with South Korea, the US, China, Japan and Russia on its denuclearization.
The other countries have since tried to persuade North Korea to return to the negotiating table.
Earlier this year North Korea offered to stop nuclear tests if the US stopped holding military drills with Seoul, but the offer was rejected.
North Korea has conducted three nuclear tests, in 2006, 2009 and 2013.