North Korea has almost completed enlargement of its main satellite launch pad, allowing the launch of rockets up to 50 metres in length as early as next month, a US think-tank said Friday.

The closely-followed 38 North website of the Johns Hopkins University's US-Korea Institute said recent satellite imagery showed gantry modifications at the Sohae launch site in northwest North Korea were almost finished.

The images revealed a new level had been added to handle rockets up to 50 metres (165 feet) in length -- almost 70 percent longer than the Unha-3 rocket which successfully put a satellite in orbit in December 2012.

That launch was condemned by the international community as a disguised ballistic missile test and resulted in a tightening of UN sanctions.

North Korea followed up the launch two months later with its third -- and largest -- nuclear test.

38 North said modification of the launch pad should be completed by March or April if work progresses at the current rate.

"The pad will then be available for additional launches, probably of the Unha-3 rocket or a slightly longer variant, such as the Unha-9, which was first displayed as a model in 2012," it said.

Active preparations for the last Unha-3 launch had begun six weeks in advance, and the website noted that no such activity had been detected so far at the modified site.

Despite UN sanctions, North Korea has vowed to push ahead with both its nuclear weapons and missile programmes.

Pyongyang already claims to have a working inter-continental ballistic missile, but one has never been tested and many experts believe that prototypes displayed at recent military parades are mock-ups.