A new satellite image shows that North Korea has made progress in building a light-water reactor to expand its nuclear programme, a private non-proliferation group has said.

The picture of the nuclear complex at Yongbyon was taken on 3 February – nearly a month before North Korea agreed to freeze major nuclear activities in return for US food aid.

The image, from a commercial satellite, was released by the Institute of Science and International Security, based in Washington, on Tuesday. Paul Brannan, a senior analyst, said a turbine building at the reactor that was still being built in September appeared to be externally complete.

North Korea says the reactor is for electricity generation, and two US academics who visited the site in 2010 and have studied subsequent satellite imagery said the reactor appeared designed for that purpose. Other experts fear it could be designed to produce plutonium for bombs.

North Korea has reprocessed spent fuel from an older reactor at Yongbyon to extract plutonium and held nuclear tests in 2006 and 2009.

Last week's US-North Korea agreement is seen as a step towards negotiations on getting the North to abandon its nuclear weapons programme in return for more aid.