Apparently new long-range ballistic missiles displayed at a North Korean military parade this month were mock-ups, according to two German experts who termed the exercise "a nice dog and pony show".

One of the missiles on show in Kim Il-Sung Square on April 15 -- transported on a launcher of apparent Chinese design -- seemed to be a new addition to the nuclear-armed country's long-range arsenal, according to analysts at the time.

But Markus Schiller and Robert H. Schmucker, of Schmucker Technologie, said all six of the road-mobile intercontinental ballistic missiles (ICBMs) on show that day were models.

"A closer look reveals that all of the presented missiles are mock-ups," they wrote in a report seen Tuesday and carried on the armscontrolwonk.com website.

"There is still no evidence that North Korea actually has a functional ICBM," they said in the April 18 report.

Schiller and Schmucker said the surface structure of the warheads was undulated, while a real warhead would have to be designed to withstand atmospheric re-entry.

The experts also said a road-mobile missile of such a size was always solid-fuelled, but those on show had parts resembling valves for liquid propellants.

Schiller and Schmucker said it would not have been possible to securely bolt the missiles to the launch tables, since the hole that might hold the bolt was aligned with the outer diameter of the missile, or very close to it.

In addition, each missile had slightly different cable duct positions and other features. "There is no doubt that these missiles were mock-ups," they wrote.

The parade featuring some 880 items of weaponry was staged to mark the centenary of the birth of founding leader Kim Il-Sung.

It came just two days after the failure of a rocket launch that the North said was designed to put a satellite into orbit.

Schiller and Schmucker said the question was whether the mock-ups were modelled on a real design that was still undisclosed, or whether the presentation was staged just for show and to secure some strategic leverage.

"Judging from other insights about the North Korean missile programme, the latter seems more likely," they wrote, recommending close monitoring of future developments.

"For now, the ICBM presentation was nothing else than a nice dog and pony show."