Lee Myung-bak didn't offer specifics about those consequences nor did he say what actions South Korea will take in response to the artillery barrage on Yeonpyeong Island that killed four, including two civilians, and sent tensions soaring in the region.
"I feel deeply responsible for failing to protect my people's lives and property," Lee said. He said he was "very sorry and regretful" for the death of innocent people.
"Launching military strikes against civilians is an inhumane crime that is even banned during wartime," Lee said.
As Lee spoke, a nuclear-powered U.S. supercarrier and a South Korean destroyer participated in joint military exercises that were a united show of force aimed at the North.
North Korea, meanwhile, threatened another "merciless" attack, as South Korean protesters begged Lee to find a way to resolve the tension and restore peace.
China, the North's only major ally, has belatedly jumped into the fray. Beijing's top nuclear envoy, Wu Dawei, called for an emergency meeting in early December among regional powers involved in nuclear disarmament talks, including North Korea.
Seoul gave a cool response to Beijing's proposal, saying it should be "reviewed very carefully" in light of North Korea's recent revelation of a new uranium-enrichment facility.
The troubled relations between the two Koreas, which fought a three-year war in the 1950s, have steadily deteriorated since Lee's conservative government took power in 2008 with a tough new policy toward the North.
Eight months ago, a South Korean warship went down in western waters, killing 46 sailors in the worst attack on the South Korean military since the Korean War. Then, last Tuesday, North Korean troops showered artillery on Yeonpyeong, a South Korean-held island that houses military bases as well as a civilian population of 1,300 -- an attack that marked a new level of hostility.
Two South Korean marines and two civilians were killed and 18 others wounded in the hailstorm of artillery that sent residents fleeing into bunkers and reduced homes on the island to charred rubble.