Kim Jong-Nam, the half brother of Kim Jong-Un who took control of the hermit state on the death of their father last month, says the military has become so powerful it will step in and take over.

The comments come in a book by Yoji Gomi, a Japanese journalist who says he built a relationship with Jong-Nam after the pair met in Beijing in 2004.

"North Korea is very unstable," Jong-Nam told Gomi, who interviewed him at length in the Chinese territory of Macau last year.

"My father governed the country with the backing of the military, but the power of the military has become too strong," he said in Korean. "If the succession ends in failure, the military will wield the real power for sure."

In the book, based on email exchanges and interviews, Jong-Nam says North Korea's troubled state-managed economy presented the regime with a dilemma.

"It is obvious that (the) economy will collapse without reforms, but the reforms will lead to a crisis of the collapse of the regime," Jong-Nam said in the interview carried out before the death of his father Kim Jong-Il on December 17.

He also claimed that his inexperienced brother Jong-Un was likely to be merely a symbol used by ruling elites to maintain their grip on power.

"Anyone with normal thinking would find it difficult to tolerate three generations of hereditary succession," he said an email, which Gomi says was sent on January 3.

"I question how a young heir with two years (of training as a successor) would be able to inherit... absolute power," he said.

"It is likely that the existing power elites will succeed my father by keeping the young successor as a symbol."

Jong-Nam has lived in virtual exile in China for many years after falling out of favour with his father, who in turn inherited the rule of the impoverished country from his own father.

Two years ago and with his health rapidly deteriorating, Kim Jong-Il moved Jong-Un -- believed to be in his late 20s -- into the position of designated successor, giving him military posts and raising his profile.

Jong-Nam did not directly respond to questions over whether he attended the elaborate funeral and memorial ceremony for his father, but Japanese media have said he visited Pyongyang after learning about his father's death.

Gomi said Jong-Nam may still take the reins of power in the secretive state with the backing of Beijing, which frets that a collapse in the regime could send millions of starving North Koreans over its border and create nuclear havoc on the peninsula.

"He has been protected by the Chinese side," said Gomi, a senior staff writer at the Tokyo Shimbun who was previously based in Beijing and in Seoul.

"If the Jong-Un regime collapses, (China) appears to be planning to send him to Pyongyang and make him become the next leader," he said.

Gomi said he had decided to go ahead with publishing the book despite requests from Jong-Nam for a delay.

"My father Kim Jong-Il and Me" will be published in Japan by Bungeishunju on Friday.