"It says, first of all, they had successful first staging and (were) able to control the rocket through staging," retired General Henry Obering told CNN television.

"That is a significant step forward for any missile program because often times the missiles become unstable as they go through the staging events," Obering said.

But the following stages failed, with part falling in the Sea of Japan and the rest in the Pacific, he told the US Cable News Network.

"The fact that they did not get apparent separation of the payload from the second or third stage means that they have more work to do there in terms of being able to achieve that," he said.

"The bottom line is they are continuing to advance in their ranges and I think it's why it's important that we have the ability to defend against these types of threats," Obering said.

North Korea launched on Sunday a Taepodong-2 missile, which normally has three stages and an estimated range of 4,100 miles (6,700 kilometers).

On July 5, 2006, North test-fired seven missiles, including a long-range Taepodong-2 which explodes after 40 seconds

He said the other six launches succeeded, which amounted to a good sales pitch.

Anybody who "is willing to buy the missiles they would be willing to sell to," he said

"The one thing in their brochure they have not been able to demonstrate is the long-range missile," he added.

North Korea has sold hundreds of ballistic missiles to Iran, Syria and Pakistan over the last decade in a bid to obtain foreign exchange, according to a study commissioned by Congress in 2007.

In December 2002, 15 North Korean Scud missiles were seized from a ship headed for Yemen.

A number of experts said however that North Korea does not yet have the technology needed to equip a missile with a nuclear warhead.

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