Fresh satellite images confirm that the Democratic People's Republic of Korea has halted construction on a facility to launch a new generation of long-range rockets, a United States research institute said on Tuesday.

The building of a new launch pad, rocket assembly building and launch control center 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 website.

The new facilities were intended to handle rockets larger than Pyongyang's Unha-3 space launch vehicle, which successfully placed a satellite in orbit this past December.

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 United Nations 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," the website 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 Pyongyang may have decided to slow or even end its efforts to build larger rockets," it said.

Such a step would mark a major policy change.

Despite international criticism and UN sanctions, the DPRK has repeatedly made it clear that it intends to pursue a missile program, which it sees as a key component of an effective nuclear deterrent.

During the recent surge in military tensions on the Korean peninsula that followed the December rocket launch and its nuclear test in February, Pyongyang warned it had the ability to deliver nuclear warheads as far as the continental US.

Most experts say such claims are exaggerated, but Washington announced in March that it was deploying new missile interceptor batteries in Alaska in order to "stay ahead" of the nuclear threat from the DPRK.

Agence France-Presse in Seoul