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. "Alternatively, a modified version of the KN-08 long-range mobile missile could launch small satellites from the pad, although this would probably require a more complex arrangement. "Rockets fired from this location … could travel over 4,000 kilometres before encountering a foreign land mass. This would allow full tests of North Korea's Musudan rocket." The Musudan is a mid-range rocket suspected to have a range of more than 3,000 kilometres, while the KN-08 has a potential intercontinental range and was first displayed on parade in April last year. The work at Sohae and a halt last year to construction at the Tonghae facility make it "unlikely that North Korea will test-fire an Unha or other space launch vehicles over the next six months", the website said. Despite UN sanctions, North Korea has repeatedly made it clear that it intends to pursue a missile programme.