The test had provided a "sure guarantee for the reliability of the inter-continental ballistic missile (ICBM) warhead re-entry," Kim was quoted as saying by the North's official news agency today.

A proven ability to build a warhead capable of standing the intense heat and vibration of atmospheric re-entry would be a major step-up in North Korea's nuclear strike capability, comprising a clear threat to the US mainland.

The latest claim follows more than a week of dire threats and warnings by Pyongyang of strikes against Seoul and Washington over ongoing South Korea-US large scale military exercises.

Kim's comments also came days after he was photographed posing with what state media said was a miniaturised nuclear warhead capable of fitting on a ballistic missile.

In order to boost the reliability of the country's strike capability, Kim said a nuclear warhead explosion and test firings of "several kinds" of ballistic rockets "will be conducted in a short time."

Tensions have been rising on the Korean peninsula ever since the North conducted its fourth underground nuclear test on January 6, followed by a long-range rocket launch a month later.

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