SEOUL (AFP) - Seoul's top official for North Korean affairs warned on Wednesday that the goal of eventual Korean reunification was being undermined by the waning interest of young South Koreans.

Unification Minister Ryoo Kihl Jae's comments came ahead of what is expected to be a highly emotional reunion on Thursday of families separated by the 1950-53 Korean War - the first such event in more than three years.

"The necessity of unification, completely taken for granted until some 10 years ago, is now being challenged by many, exposing the problems in our education system," Mr Ryoo said in an address to an academic forum.

"We need to strengthen unification education dramatically at primary, middle and high schools," he said.

Since the war sealed the division of the peninsula, reunification has long been the stated priority for both Seoul and Pyongyang.

But pro-merger sentiment in the prosperous South, especially among young people with no memory of the unified peninsula, has waned considerably in recent years.

A 2010 survey of South Korean teenagers showed 57 per cent had an interest in reunification - compared to 71 per cent in 1997.

With the South's economy - Asia's fourth-largest - nearly 40 times larger than that of the North, many cite concerns over the enormous financial burden of integration and the social chaos that might follow.

But Ryoo argued that unification - enshrined in the South's constitution as a national mandate - remained a moral imperative.

"Now we have no choice but to cooperate with the North... so that we can improve the lives of ordinary North Koreans," he said.

The minister also stressed the need to update the South's three-stage roadmap for reunification set in the 1990s.

The existing plan envisages reconciliation and cooperation, followed by a transitional confederation and eventual reunification.

"It sets the process of reconciliation and cooperation way too long," Mr Ryoo said, without elaborating.

"We need to make an agreement next year to come up with a different blueprint," he added.

Because the Korean conflict concluded with an armistice rather than a peace treaty, the two Koreas technically remain at war and direct exchanges of letters or telephone calls are banned.

Seoul and Pyongyang agreed earlier this month to hold a reunion event for family members divided by the war.

A group of 82 elderly South Koreans - chosen in a highly competitive lottery - are set to cross the border on Thursday to attend the gathering at a resort in Mount Kumgang.