North Korea has undertaken major construction work at its main missile launch site, possibly to cater to larger and more mobile weapons, a US think-tank said Tuesday.

Satellite images taken earlier this month suggest construction of a second flat mobile launch pad at the Sohae missile site, the US-Korea Institute at Johns Hopkins University posted on its 38 North website.

Work has also been carried out on Sohae's main launch pad, "possibly to upgrade that facility to handle future larger rockets," the post said.

The pad was used for the launch in December of the North's Unha-3 carrier, which successfully placed a satellite in orbit.

The launch was condemned by most in the international community as a disguised ballistic missile test that violated UN sanctions on Pyongyang.

North Korea insisted it was a purely scientific mission and vowed to push ahead with similar launches in the future.

"Activities related to the upgrading of the Unha launch pad may be completed soon, allowing Pyongyang to proceed with another space launch should it decide to do so," the institute said.

North Korea is currently pushing for a resumption of six-party talks on its nuclear programme, but the United States says it must first demonstrate a commitment to denuclearisation.

Another long-range rocket launch would be taken as a step in the opposite direction and almost certainly result in fresh sanctions.

Last week, the US-Korea Institute said satellite images showed North Korea had built two tunnel entrances at its nuclear test site in a sign it plans more detonations.