North Korea may have moved the first stage of a long-range rocket on to its launch pad, ahead of its possible lift-off next week, according to a U.S. specialist website.

The 38 North website (38north.org) said an April 4 photo of the launch site at Tongchang-ri in the country's northwest indicated the first stage of the Unha-3 rocket, while not visible, may be placed in the gantry.

The impoverished communist state says its rocket will put a peaceful satellite into orbit but the United States, South Korea and other nations see it as a pretext for a long-range missile test banned by the United Nations.

The website said that commercial satellite photographs reveal the gantry's work platform is now covered and closed around the mobile launch stand, indicating work is being conducted inside.

While this makes it impossible to determine whether the Unha-3 rocket or any part of it is erected on the pad, evidence suggests that the first stage has been placed as an expected launch nears, 38 North said.

Recent fuelling activity seems to have been completed, with most of the empty fuel and oxidizer tanks apparently having been removed from buildings that supply the first stage, it said.

Security has also been tightened with a barricade or security checkpoint set up for vehicles entering from the west, the only road to the launch pad from other main facilities at the range.

The launch pad itself has been cleared, with objects previously seen near the gantry now removed, it said.

The website, however, did not publish the satellite photos obtained from Geoeye on April 4. The satellite provider subsequently withdrew those pictures from its public catalogue and asked that they not be published, it said.

North Korea is preparing mass celebrations to mark the 100th anniversary on April 15 of the birth of Kim Il-Sung, the country's first and "eternal" president and founder of the dynasty which has ruled uninterrupted since 1948.

A successful satellite launch would burnish the image of his grandson Kim Jong-Un as he seeks to establish his credentials as a strong leader. (AFP)