Update | Satellite spots construction at North Korean rocket launch site

Fresh satellite imagery shows North Korea has embarked on a major new construction programme at the facility where it launched a long-range rocket in December, a US research institute said.

The construction includes what could be a new launch pad for testing mobile ballistic missiles, the US-Korea Institute at Johns Hopkins University posted on its 38 North website on Friday.

Work has been under way at the Sohae Satellite Launching Station, or Tongchang-ri, since mid-year, it said.

The station near the northwest coast was the base for the country's successful Unha-3 rocket launch in December, which was widely condemned in the West as a disguised test of a long-range ballistic missile banned under United Nations resolutions.

As well as a new road, the rebuilding of a compound for troops and the restart of construction on a permanent radar tracking system, the new satellite imagery appears to show construction of a new launch pad, the website said.

"While it is too early to identify the exact purpose of this site, one possible explanation is that Pyongyang is building a 'flat launch pad', a large concrete area that would be used to test mobile ballistic missiles fired from a transporter-erecter launcher," it said.