“It’s necessary to call China to account for maintaining an alliance with Kim Jong-Il,” Wednesday’s Chosun Ilbo newspaper quoted Hwang Jang-Yop as saying.

Hwang, former secretary of the North’s ruling communist party and an ex-tutor of Kim, defected during a trip to Beijing in 1997 and now lives under police guard in the South.

“Reunification can be discussed in 15 to 20 years if North Korea opens its doors and carries out Chinese-style reforms, and if South Korean funds and engineers are allowed access there,” the newspaper quoted him as saying at a gathering Tuesday.

Hwang said living standards in the impoverished North would improve dramatically within 15 years after a market-opening, but economic reforms were also tantamount to getting rid of absolute rule.

“It is China in the end that decides Kim Jong-Il’s fate. South Korea must sign a free trade pact with China to draw China and North Korea apart,” he was quoted as saying.

Hwang described Kim as a “national traitor” and added: “At the moment, no Chinese leaders think favourably of Kim Jong-Il.”

He blamed the “sunshine” aid and engagement policy of previous liberal governments in Seoul for the slow pace of change in the North. The current conservative government has dropped the policy.

“When I left, I predicted that North Korea would not survive for another five years. But the prospects have darkened,” the 86-year-old was quoted as saying.

China is North Korea’s only major ally, and its largest trade partner and energy supplier.

Global investment bank Goldman Sachs, in a research paper published this week, said the economy of a unified Korea could be larger than France, Germany and possibly Japan in 30-40 years if the North’s growth potential was realised.

It highlighted the North’s abundant and competitive workforce, mineral resources worth 142 times the current annual gross domestic product and “significant room for productivity gains.”