Satellite images reportedly show large-scale construction work in North Korea believed to be aimed at advancing its intercontinental ballistic missile technology.

NK News, which carries news on North Korea, says photos taken this month appear to show workers installing roofing over a new manufacturing building at the Kusong Tank Plant in the country's northwest.

Pyongyang launched an ICBM-class Hwasong-14 missile from Kusong in July 2017.

An expert told NK News that in recent years the factory has appeared to be involved in manufacturing new tracked transporter erector launchers.

He added that the launchers provide an alternative to wheeled vehicles for carrying heavier missiles introduced in the last five years over rough terrain.

Meanwhile a US think tank, the Center for Strategic and International Studies, on Wednesday released the results of an analysis of imagery captured this month of another North Korean facility.

The CSIS said the photos show "dramatic and ongoing construction progress throughout the eastern half of the Sohae Satellite Launching Station." The complex is located at Tongchang-ri in the country's northwest.

The CSIS says the "activity has expanded beyond the launch pad area" over the past two months.

A North Korean media outlet reported in March that leader Kim Jong Un called for modernizing the facility to enable the launch of large carrier rockets, including one for a military reconnaissance satellite.

North Korea launched what were believed to be long-range ballistic missiles from the site in 2012 and 2016. It described them as earth observation satellites.

The CSIS says the modernization plans at the facility will support the development of technology that can also be used in North Korea's ICBM programs.