SEOUL, South Korea — South Korea’s military announced provocative new artillery drills on the front-line island shelled in a deadly North Korean attack, then immediately postponed them Monday in a sign of disarray hours after the president vowed to get tough on the North.

Similar live-fire maneuvers by South Korean troops one week earlier triggered the North’s bombardment that decimated parts of Yeonpyeong Island, killed four people and drew return fire in a clash that set the region on edge.

The new drills originally planned for Tuesday could have had even higher stakes: South Korean and American warships are currently engaged in separate military exercises in waters to the south.

Officials at the Joint Chiefs of Staff told The Associated Press on Monday that the latest drills were postponed after the marine unit on the island mistakenly announced them without getting final approval from higher military authorities. The cancellation had nothing to do with North Korea, and the drills will take place later, one official said. The officials spoke on condition of anonymity, citing agency rules.

Pyongyang had warned last week that it would consider any South Korean drills off Yeonpyeong Island a deliberate provocation and territorial violation, and urged Seoul to call off last week’s exercises. The artillery attack that came after South Korea went ahead with its drills killed four and injured 18 people.

Earlier Monday, President Lee Myung-bak gave his first address to the nation in nearly a week, taking responsibility for failing to protect his citizens, expressing outrage at the North’s “ruthlessness” and vowing tough consequences for any future aggression.

Lee has come under withering criticism for what opponents have called lapses in South Korea’s response to the attack just eight months after the sinking of a South Korean warship in nearby waters that killed 46 sailors.

Hours after his speech, authorities on Yeonpyeong (yuhn-pyuhng) announced new live-fire drills for Tuesday morning, warning residents by loudspeaker to take shelter in underground bunkers well in advance. Another announcement later in the evening said there would be no exercise.

Meanwhile, a nuclear-powered U.S. supercarrier and a South Korean destroyer carried out joint military exercises in the waters south of the island in a united show of force by the longtime allies.

Jets roared off an aircraft carrier as part of drills to practice air defense, combat warfare and search-and seizure drills, said Rear Adm. Dan Cloyd, commander of the USS George Washington Carrier Strike Group.

Lee observed the drills Monday from the U.S. Army’s command center in Seoul, along with Gen. Walter Sharp, the top commander in South Korea, in a pointed show of unity.

“This exercise demonstrates our interoperability and how closely integrated the ROK-US forces are along with the capability both bring in defense of the peninsula that is the cornerstone of regional stability,” Sharp said in a statement posted by the military, using the acronym for the Republic of Korea, the South’s formal name.

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 on Monday.

“Only a few meters (yards) 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.”

In the past week, Lee has replaced his defense minister, ordered reinforcements for the 4,000 troops on Yeonpyeong and four other Yellow Sea islands, and upgraded the military 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 warned.

Minutes later, North Korea issued another threat to attack South Korea and the United States, calling the allies’ joint war drills “yet another grave military provocation.”

The two Koreas are required to abide by an armistice signed in 1953 at the close of their brutal, three-year war.

However, North Korea does not recognize the maritime border drawn by the U.N. at the close of the war, and considers the waters around Yeonpyeong Island — just seven miles (11 kilometers) from its shores — its territory.

The waters have been the site of three deadly skirmishes since 1999, as well as last week’s artillery attack and the sinking of the Cheonan warship.

Yeonpyeong Island, normally home to about 1,300 civilian residents, was declared a special security area Monday, which could pave the way for a forced evacuation of those left on the island.

Military trucks carrying what appeared to be multiple rocket launchers were seen heading to a marine base on the island Monday.

Long-range artillery guns and a half-dozen K-9 howitzers were also on their way, the Yonhap news agency reported, citing unnamed military officials.

China, North Korea’s only major ally, sought to calm tensions. Beijing’s top nuclear envoy Wu Dawei, has called for an emergency meeting among regional powers involved in nuclear disarmament talks, including North Korea.

Seoul, which wants proof of Pyongyang’s commitment to denuclearization as well as a show of regret over the Cheonan incident, reacted coolly to the proposal.

In Tokyo, Japan’s Prime Minister Naoto Kan reiterated Monday that his government would work with Seoul and Washington on how to counter North Korea’s “reckless” acts.

Yeonpyeong Island was a charred, wrecked shell of the former fishing community it was before last week’s artillery attack. Only 300 people are left, including a few residents, many more journalists, and some officials.

Among those left on the island was a band of orphaned dogs, rescued Monday by animal rights activists. One puppy with floppy ears peeked out of a rescuer’s backpack while being carried across a plank onto the boat to the mainland.

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AP photographers David Guttenfelder and Lee Jin-man on Yeonpyeong Island, Kelly Olsen in Seoul, and Mari Yamaguchi in Tokyo contributed to this report.