Fresh satellite images show substantial new construction at North Korea's space rocket launch site, mirroring leader Kim Jong-Un's recent vow to launch more satellites in defiance of UN resolutions, a US think-tank said Friday.

The impoverished but nuclear-armed North sent an Unha-3 rocket into space in December 2012 from the Sohae satellite launch site -- a move the international community condemned as a disguised ballistic missile test and punished with further sanctions.

Two months later, the North conducted its third -- and most powerful to date -- nuclear test.

Using satellite analysis, the US-Korea Institute at Johns Hopkins University had reported last year a substantial upgrade at Sohae -- allowing it to handle rockets up to 50 metres (165 feet) in length -- almost 70 percent longer than the Unha-3.

In its latest assessment of images taken in mid-May, the institute noted further modifications, including construction of a new support building next to the Unha-3 launch pad and a rail mobile platform between the two.

While the precise purpose of the building was not immediately clear, institute analyst Tim Brown said it could be a facility for assembling a launch vehicle and then rolling it to the launch pad.

There has been speculation that the North might launch a long-range rocket to mark the 70th anniversary of the ruling Workers' Party in October -- a schedule Brown labelled "difficult although not impossible".

The ongoing upgrade at Sohae represents a "significant" investment, Brown said, and is "another indicator ... that the North is determined to pursue its space programme."

Visiting a newly-built satellite command centre earlier this month, Kim vowed to push ahead with further satellite launches to confirm the country's space power status.

"Space development can never be abandoned, no matter who may oppose it," Kim said.

The successful launch of a new, larger rocket would fuel mounting concern over the expansion of the North's nuclear weapons and ballistic missile programmes.

Pyongyang recently hailed the "historic" test of a submarine-launched ballistic missile, while boasting of its ability to miniaturise a nuclear warhead to fit on high-precision, long-range rockets.

Experts question both claims, but broadly agree that the North is moving ahead quickly with the development of both programmes.

Meeting in Seoul on Wednesday, the nuclear envoys of South Korea, the United States and Japan highlighted the seriousness of Pyongyang's progress.

"Their intention is clear, and we should be concerned regardless of the stage of their development," said US envoy Sung Kim.