An emotional President Lee Myung-bak vowed Monday to respond "resolutely and unwaveringly" against those behind the deadly sinking of a naval ship near the tense maritime border with North Korea.

Seoul has not openly blamed Pyongyang for the March 26 explosion that broke the 1,200-ton Cheonan in two during a routine patrol, killing at least 38 of the 104 sailors on board the ship. Fifty-eight were rescued and eight remain missing.

However, officials said they were investigating the possibility that a North Korean naval mine or torpedo struck the warship. The chief investigator said Friday that an external explosion appeared more likely than an explosion of munitions stored inside the ship or the vessel hitting an underwater rock.

"I promise you that as president, I will uncover the cause of the Cheonan's sinking down to the very last detail," Lee said in an emotional 10-minute televised speech early Monday.

North Korea, making its first official comments about the incident, denied involvement Saturday and accused the South of spreading false rumors.

The two Koreas technically remain in a state of war since their 1950-53 Korean War ended in a cease-fire, not a peace treaty. North and South Korea have fought three bloody sea battles in the Yellow Sea.

Lee did not mention North Korea in his address, but vowed to deal "resolutely and unwaveringly" with the outcome of the investigation. He pledged to build a stronger military to ensure such an incident never happens again.

Wiping away tears with a handkerchief, he read out the names of all the dead and missing sailors. The disaster is among the worst in South Korea's naval history.

"The country that you loved will never forget any of you," he said.

Pyongyang's official state media quoted a military commentator as criticizing Seoul for linking North Korea to the blast. The commentator accused Seoul of seeking to shore up sanctions against the North and to muster conservative votes for upcoming mayoral and gubernatorial elections.

"It is a trite trick of the stupid to hatch plots and stoop to any infamy under that pretext whenever they are driven into a tight corner," the report from the Korean Central News Agency said.

The U.N. Security Council slapped tough new sanctions on North Korea following a nuclear test, its second, last year.

South Korea's foreign minister said Seoul would take the issue to the Security Council if North Korea emerges as the culprit behind the sinking of the Cheonan.

Foreign Minister Yu Myung-hwan said it was too early to determine the cause but that the government was bracing for all possible outcomes.