A US research group says recent satellite imagery indicates a high level of activity around a nuclear complex in North Korea.

The group, known as 38 North, on Saturday released its analysis of satellite photos taken last month of the Nyongbyon facility in the country's northwest.

The group says water discharges have been detected that could be associated with testing of the cooling system for an experimental light water reactor. It says this may indicate the reactor is nearing completion.

The group also says new construction started around a uranium enrichment plant in February.

North Korea's state-run TV reported last week that leader Kim Jong Un issued a directive to increase the production of weapons-grade nuclear materials and expand the nuclear weapons arsenal.