SEOUL, South Korea -- South Korea's president on Monday took responsibility for failing to protect his citizens from a deadly North Korean artillery attack last week, vowing tough consequences for any future aggression and expressing outrage over the "ruthlessness of the North Korean regime."

Lee Myung-bak's short speech to the country came as a nuclear-powered U.S. supercarrier and a South Korean destroyer participated in joint military exercises, a united show of force nearly a week after an artillery barrage on Yeonpyeong island killed four, including two civilians.

Amid the heightened tension, classified U.S. State Department documents leaked Sunday by online whistle-blower WikiLeaks showed the United States and South Korea discussing possible scenarios for reunification of the peninsula, and American worry over Pyongyang's nuclear weapons program.

Under pressure to take stronger action in dealing with the defiant North, Lee lashed out at Pyongyang.

"Only a few meters away from where shells landed, there is a school where classes were going on," Lee said. "I am outraged by the ruthlessness of the North Korean regime, which is even indifferent to the lives of little children."

Lee has come under withering criticism for what opponents have called lapses in South Korea's response to the attack. Lee has replaced his defense minister, ordered reinforcements for the 4,000 troops on Yeonpyeong and four other Yellow Sea islands and upgraded rules of engagement.

"If the North commits any additional provocations against the South, we will make sure that it pays a dear price without fail," Lee said. "The South Korean people now unequivocally understand that prolonged endurance and tolerance will spawn nothing but more serious provocations."

He didn't offer specifics about what consequences the North would face, and he offered few details on what actions South Korea will take in response to last week's attack, other than promising to strengthen the military.

"I feel deeply responsible for failing to protect my people's lives and property," Lee said. In South Korea, it is not rare for top officials to resign, apologize or express responsibility when their government faces public criticism.

Minutes after Lee finished his speech, North Korea issued a fresh threat to attack South Korea and the United States, calling the allies' joint war drills "yet another grave military provocation."

The maneuvers are an "intentional plot" by the United States and South Korea to prepare for war against North Korea, Pyongyang's main Rodong Sinmun newspaper said in a commentary.

The North will launch counter attacks without hesitation on South Korea and U.S. forces if they engage in provocation again, according to the commentary carried by the official Korean Central News Agency.

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 the 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 -- 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.

North Korea blamed the South for provoking the attack by holding artillery drills near the Koreas' maritime border, and has threatened to be "merciless" if the war games -- set to last until Dec. 1 -- get too close to its territory.

Appearing on CNN's "State of the Union" on Sunday, U.S. Sen. John McCain said it was time to discuss "regime change" in North Korea, but the former Navy combat pilot didn't say how he advocates changing its government.

McCain, the top Republican on the Senate Armed Services Committee, said he was not suggesting military action against the North. He said the Chinese, the North's closest ally, should rein in its neighbor, and he accused Beijing of failing to play a responsible role in either the Korean peninsula or the world stage.

North Korea has walked a path of defiance since launching a rocket in April 2009 in violation of U.N. Security Council resolutions and abandoning the disarmament process in protest against the condemnation that followed.

However, in recent months Pyongyang has shown an eagerness to get back to the talks, and has appeared increasingly frustrated by U.S. and South Korean reluctance to restart the negotiations.

Seoul has said it wants an acknowledgment of regret for the sinking of the Cheonan warship in March as well as a concrete show of commitment to denuclearization.

North Korea, which cites the U.S. military presence in South Korea as a main reason behind its drive to build atomic weapons, routinely calls the joint exercises between the allies a rehearsal for war.

Washington, which keeps 28,500 troops in South Korea to protect the ally, insists the routine drills were planned before last Tuesday's attack.

The documents leaked by WikiLeaks showed deep U.S. worries about North Korean and Iranian nuclear programs.

The New York Times published documents that indicated the United States and South Korea were "gaming out an eventual collapse of North Korea" and discussing the prospects for a unified country, if the North's economic troubles and political transition lead it to implode.