SEOUL • Satellite imagery has revealed what appeared to be the installation of a reviewing stand for foreign media invited to observe the planned decommissioning this week of North Korea's only known nuclear test site, according to a US website.

The respected 38 North website said in a post dated May 18 that additional steps have been taken at the Punggye-ri nuclear test site towards its closure, including "preparations to build a safe reviewing stand for visitors".

Four rows of objects previously identified near the west portal have been significantly altered and have increased substantially in height, according to the article by experts Frank V. Pabian, Joseph S. Bermudez Jr and Jack Liu.

Apart from the observation platform to allow journalists to safely view the blowing up of the site's tunnels, satellite imagery from May 15 also showed a pathway leading from a dirt road below to the platform.

The location of the stand could provide a clear view of the tunnel entrances to be blown up, said the 38 North website.

Separately, a government source has told Yonhap news agency that the North is apparently making preparations to transport foreign journalists to the nuclear test site.

The source said North Korea has shown signs of restoring sections of a decrepit railway link between the eastern coastal city of Wonsan and Kilju in North Hamgyong province, where the Punggye-ri site is located.

There are also signs of test train runs along the 270km route, reported Yonhap.

The Punggye-ri site consists of a system of tunnels dug beneath Mount Mantap in the country's north-east. It is the site of all six of the North's nuclear tests, the latest and by far the most powerful in September last year, which Pyongyang said was an H-bomb.

In an earlier post dated May 14, the 38 North website said satellite images dated May 7 showed "the first definitive evidence that dismantlement of the test site was already well under way".

Several key operational buildings as well as smaller sheds had been razed and rails connecting the tunnels to their waste piles were removed, the monitoring group said.

Excavation of a new tunnel has also been halted since late March, it added.

North Korea pledged to close the testing ground after its leader, Mr Kim Jong Un, last month declared the country's nuclear force complete and said it had no further need for the complex.

Its Korean Central New Agency said dismantlement of the nuclear test ground would involve collapsing all of its tunnels with explosions, blocking its entrances, and removing all observation facilities, research buildings and security posts.

Reporters from South Korea, the US, China, Russia and the UK have been invited for the decommissioning ceremony, scheduled between Wednesday and Friday.

But a recent row with Seoul over its joint military exercise with the US has also raised questions over the attendance of South Korean media.

As of last Friday, the North had not responded to the list of eight South Korean journalists tapped to cover the ceremony.

South Korea tried to notify North Korea of their names through a hotline at the truce village of Panmunjom, but the North did not respond.

The lack of response prompted the South and the absence of observers from international atomic monitoring agencies at this week's ceremony raised concerns over Pyongyang's intent and the openness of the process.

A North Korean propaganda website yesterday hailed the decommissioning of the Punggye-ri site as "a very meaningful and significant measure" undertaken voluntarily by Pyongyang to achieve the denuclearisation of the Korean Peninsula based on the spirit of an agreement reached at the historic inter-Korea summit last month.

It slammed conservative forces in the South, including the main opposition Liberty Korea Party, for underestimating the planned closure of the site.