The building of a new launch pad, missile assembly building and launch control centre at the Tonghae Satellite Launching Ground slowed and then stopped around September 2012.

"While it was expected that construction would continue this spring, new imagery indicates that work had not resumed as of late May 2013, almost eight months later," the US-Korea Institute at Johns Hopkins University posted on its 38 North web site.

The new facilities were intended to handle rockets larger than the North's Unha-3 space launch vehicle (SLV) which successfully placed a satellite in orbit in December last year.

Although tagged by Pyongyang as a purely scientific mission, that launch was seen by the international community as a disguised ballistic missile test and was punished by additional UN sanctions.

The 38 North website said it was unclear why work at the Tonghae site had been halted.

Initial speculation that it was due to construction teams being sent elsewhere to repair widespread typhoon damage now seems unlikely given the eight-month time lapse, it added.

"If work resumes, completion of the new launch facilities ... would appear to be at least a year behind the estimate of their original (2016) schedule" it said.

The website speculated that Pyongyang may have decided that one launch pad capable of handling larger rockets than the Unha -- located at the more modern Sohae facility -- was sufficient to press ahead.

"An alternative explanation is that the North may have decided to slow or even end its effort to build larger rockets," it said.

Such a step would mark a major policy change.