Video footage of North Korea's latest missile test appears to show it breaking up before landing, indicating that Pyongyang may not yet have mastered re-entry technology needed for an operational nuclear-tipped missile, a think-tank reported on Monday.

The apparent failure of the re-entry vehicle in Friday's test could mean North Korea will need to carry out several more tests of its intercontinental ballistic missile before it can be deemed operational, missile expert Michael Elleman told reporters.

Elleman, an expert with the International Institute for Strategic Studies, cited video footage taken by a weather camera in Japan's Hokkaido Prefecture broadcast by Japan's NHK television.

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In a report for 38 North, a Washington-based North Korea monitoring project, Elleman said the video showed the re-entry vehicle shedding small radiant objects at an altitude of four or five kilometres before dimming and disappearing.

"Had the RV survived the rigours of re-entry, it would have continued to glow ... A reasonable conclusion based on the video evidence is that the ... re-entry vehicle did not survive," he said.

Even so, Elleman said, further testing could still allow North Korea to deploy an operational ICBM next year.

US officials say the Defense Intelligence Agency, the Pentagon spy agency, has determined that North Korea will be able to field a reliable nuclear-capable ICBM by next year, earlier than previously thought.

North Korea said on Saturday its second ICBM test, which followed its first on July 4, had proven its ability to strike the whole of the US mainland.

Some US-based experts said last week's launch showed the missile could have been capable of going as far into the United States as Denver and Chicago, with New York and Washington just out of range.

However, John Schilling, another missile expert and 38 North contributor, said last week the improved performance over the July 4 test could have been the result of using a lighter payload to improve its range.

Elleman said the size of the payload was crucial to range and a lighter payload could mean a more fragile re-entry vehicle less able to withstand re-entry.

He estimated the current missile was probably capable of carrying a 500kg bomb of similar power to those used at Hiroshima and Nagasaki in World War II as far as the US West Coast.

Source: Reuters news agency