South Korean President Lee Myung-bak delivers a speech to the nation during his New Year's speech at the presidential Blue House in Seoul, South Korea, Monday, Jan. 2, 2012. (Jin Sung-chul/AP)

Saying the Korean Peninsula was “at a turning point,” South Korean President Lee Myung-bak on Monday offered North Korea a “window of opportunity” to improve relations but warned of a powerful retaliation if Pyongyang launches another military strike.

“There should be a new opportunity amid changes and uncertainty,” Lee said. “If North Korea shows its attitude of sincerity, a new era on the Korean Peninsula can be opened.”

In his first major policy speech since the Dec. 17 death of Kim Jong Il, Lee did not specifically mention new North Korean supreme leader Kim Jong Eun, the young hereditary heir. But he offered Kim Jong Eun largely the same nuclear weapons-for-aid bargain that Kim Jong Il spent years rejecting. Any change on the peninsula, then, will come from a new strategy in Pyongyang, experts said — not a new strategy in Seoul.

Entering the final year of a five-year fixed term, Lee has tried to push North Korea toward an economic leap by offering massive aid and investment — if the authoritarian nation gives up its weapons. Lee’s stance, which reversed a 10-year policy of unconditional aid, has coincided with a tense and bloody era of intra-Korean relations, with a pair of 2010 attacks killing 50 South Koreans.

Lee on Monday repeated his stance that multi-nation nuclear talks can resume if Pyongyang first pledges to freeze its nuclear activities, as it promised in earlier — and now ignored — agreements. Talks have been on hold since the North walked out almost three years ago.

The upcoming year, Lee said, “will set a milestone for resolving the North Korean nuclear issue.” He did not elaborate on his reasoning.

The North recently has sent mixed messages about the necessity of its nuclear weapons, which include a small stockpile of plutonium and a more modern uranium enrichment program. North Korea in March said the country would never make the mistake of Libya, which abandoned its nuclear program in 2003. Saturday, several powerful political bodies published a message describing the North as a “nuclear state with unrivalled military strength no enemy would dare challenge.”

But just days before Kim Jong Il’s death, the North was nearing a reported deal to swap food aid for a freeze in its uranium enrichment program. That deal is now on hold, as neighboring countries figure out how to handle North Korea’s new leadership. It’s also unknown whether Kim Jong Eun, or his cadre of older advisers, will be open to such a trade-off at a time when the country, trying to build support for its leadership transition, has become newly vulnerable.

Some security experts fear that North Korea could try to bolster its unity by lashing out against the South. Monday, Lee reiterated a promise he first made in the wake of the November 2010 shelling of a South Korean border island in the Yellow Sea: If the North attacked, the South would respond forcefully.

“As long as there continues to be a possibility of North Korean provocation,” Lee said, “we will maintain a watertight defense posture.”

Lee’s hard-line stance toward the North doesn’t figure to be the top issue among South Koreans in this year’s presidential election, because most are more worried about inflation, a widening income gap and runaway education costs. Still, South Korea is polarized by its North Korea policy, and a provocation in advance of the presidential poll in December could cause a wild — and difficult-to-predict — shift in what the public wants.

“Although voters tend to favor more hawkish policies at times of insecurity,” a recent report by the International Crisis Group said, “the right in the South is facing the paradox that voters may blame Lee’s tough line for the increased tensions. ... The North Korean leadership could calculate that rising tensions will push the South Korean electorate towards candidates who favor a more conciliatory policy.”