Earlier this month at a parade in Pyongyang, the North Korean authorities caught the attention of a number of western experts by displaying what appeared to be six road-mobile inter-continental ballistic missiles.

It was the first time that this system had been seen in public and it appeared to mark yet another step forward in North Korea's ambitious missile development programme.

But now, after a careful study of pictures from the parade, a team of German arms experts thinks the missiles are not quite what they seem.

The North Korean authorities clearly intended the parade to impress onlookers with a display of military might.

"The displayed missiles all have minor differences in detail that add up to major differences in configuration and design. You will never see this in real missiles." Markus Schiller, German missile expert

The six new long-range missiles carried on top of massive, wheeled transporter vehicles certainly provoked interest.

For a start many analysts wondered where these giant vehicles had come from.

There were strong indications that they may actually have been supplied by a Chinese company. Beijing might well be embarrassed to be seen to be supporting Pyongyang's missile development programme.

But now attention has switched back to the missiles themselves, for, after a careful study of images from the parade, two German missile experts, Markus Schiller and Robert Schmucker, have published a paper suggesting that the North Korean missiles are fakes - elaborate mock-ups and not the real thing.

'Something wrong'

Markus Schiller explained why the North Korean parade had aroused his suspicions.

"If you take a close look at the displayed missiles," he said, "there are many details that are plainly wrong, indicating that there might be something wrong with the whole programme.

North Korea's missiles: Mock-ups or real?

1. The markings on each of the missiles are different. Some possibly show filling or draining valves for liquid propellant - inconsistent with other design features that suggest a solid fuelled system. 2. The missiles are not perfectly aligned to the launch table. This suggests it is hard to bolt the missile down in launch configuration. 3. Undulations in the surface metal of the warhead suggest thin sheeting over stringers or formers (supports) unlike the more substantial construction required of a real warhead. Source: Markus Schiller, Robert H. Schmucker

"Missiles are highly complex weapons. Unlike a satellite launcher, you do not build a prototype that you launch once and then incorporate all the lessons learned into the next prototype.

"With missiles, you produce a whole batch of identical systems, usually with a proven design. Otherwise you have no idea whether the weapon will really work when push comes to shove.

"The displayed missiles all have minor differences in detail that add up to major differences in configuration and design. You will never see this in real missiles," he said.

His conclusion: The missiles paraded by the North Koreans were mock-ups and for now at least, Pyongyang does not have a functioning intercontinental ballistic missile.

A detailed analysis of the weapons show several problem areas. For one thing, there is no obvious line of separation between the warhead and the third stage of the missile.

Showing mock-ups at parades actually is a common practice Markus Schiller, German missile expert

There are also basic questions about how it is fuelled. At first glance it looks like a solid-fuel system, which is what you would expect for a road-mobile system.

"But," Markus Schiller said, "you can see markings on the stages that look like propellant fill or drain valves - you don't have that for solid-fuelled rockets. And so far, all of the bigger North Korean missiles were liquid-fuelled.

"The technology is totally different, perhaps comparable to a piston engine and a gas turbine, or an electric motor. Therefore, it seems consistent that this missile is liquid-fuelled, and not solid-fuelled.

But he adds: "As long as we have never seen one of these missiles lift off, there still is a third possibility: The missile is neither liquid- nor solid-fuelled. There simply is no missile."

'Common practice'

In fairness, the two German experts do note that showing mock-ups of weapons at parades is not an exclusively North Korean deception.

Markus Schiller said that in a way, he was surprised at people's surprise.

"Showing mock-ups at parades actually is a common practice," he said. "We know that only inert training devices were presented at East German parades, and I am pretty sure that they did exactly as they had learned from their Soviet brothers."

Image caption If the missiles are fake, it is unclear what motive the North's leadership might have had in showing them off

"You also see mock-ups at parades in Iran or Pakistan, for example. The missiles that North Korea displayed in 2010 - the Musudan and the Nodong - also were mock-ups, but of even worse quality than the new KN-08 mock-ups."

So, why show such mock-ups at all?

"Because real missiles are precious and could be damaged during parades," Markus Schiller said.

But another motive, he suggests, might be quite explicitly "to change some details and drive the other side's analysts crazy, or to pretend that you have something that you actually don't have. There is a very long history of this last aspect."

Other analysts agree that North Korea has shown mock-ups of missiles in the past, but that eventually this technology has appeared in real weapons.

So are the six dubious missiles trundled out in this month's parade a pointer to what may be coming, or an elaborate ruse to confuse western defence analysts?

Either way, the North Koreans have certainly attracted those experts' attention.