SEOUL, South Korea - President Lee Myung-bak said Monday that South Korea is ready to respond positively if North Korea shows sincerity in dealing with its nuclear program, inter-Korean relations and the human rights of its own people.

The South Korean leader was speaking a day after North Korean heir apparent Kim Jong Un made his diplomatic debut at Pyongyang celebrations marking the 65th anniversary of the ruling Workers' Party. The lavish event further signaled that he is in line to someday succeed his father Kim Jong Il as leader of the authoritarian country.

Relations between the Koreas have been fraught since Lee took office in early 2008 with a policy of linking South Korean aid to progress in North Korea's denuclearization.

North Korea reacted angrily and relations soured, hitting their lowest point in years when Seoul accused Pyongyang of torpedoing a South Korean warship and killing 46 sailors. North Korea denies involvement in the March sinking.

Speaking to reporters from foreign news organizations at his official residence, Lee said that the younger Kim's emergence shows that North Korea is likely embarking on a generational transfer of power, but that what matters most is the country's attitude going forward.

"No matter how the third-generation succession process goes, what concerns us is North Korea having an interest in being sincere regarding the nuclear issue, inter-Korean peace and its people's human rights and happiness." Lee said.

"If North Korea shows a sincere attitude, we will always respond with an open heart," he said.

Lee's two liberal predecessors, Kim Dae-jung and Roh Moo-hyun, pursued a "sunshine policy" of good relations with the North. They seldom criticized the country and pursued joint tourism and business projects with the aim of fostering an environment of goodwill they hoped would lead to eventual peace on the divided peninsula.

Both presidents traveled to Pyongyang for summits with Kim Jong Il. Kim went in 2000 and Roh in 2007. Lee has never held a summit with Kim, though there have been reports that the two sides have discussed the possibility.

Some reconciliation projects pursued under Kim Dae-jung and Roh have withered amid the tensions. Tours from South Korea to a mountain resort in the North were suspended after a South Korean tourist was shot by a North Korean soldier in 2008.

Inter-Korean relations have shown some signs recently of warming with the two sides agreeing to hold the first reunions in a year later this month for families separated by the Korean War.