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

Hours after Lee Myung-bak's nationally televised speech, South Korea's military announced it would conduct new drills — including live-fire exercises — on Yeonpyeong Island on Tuesday morning, and issued a warning over loudspeakers for residents to take shelter in underground bunkers by 9:30 a.m.

Artillery rounds will be fired into the waters southwest of Yeonpyeong Island, not toward North Korea, an official at the Joint Chiefs of Staff said Monday. He spoke on condition of anonymity, citing department policy.

Similar live drills by South Korean troops one week earlier triggered the deadly bombardment of artillery by North Korea that decimated parts of Yeonpyeong Island, which lies just 7 miles (11 kilometers) from North Korean shores. Two marines and two civilian construction workers were killed, and 18 others injured.

The renewed live-fire drills may serve to underline Lee's attempts to be more forceful with the North, or be a move to test new weapons being installed on the island.

The military has added long-range artillery guns, doubling the amount of K-9 howitzers to 12, and multiple rocket launchers on Yeonpyeong, the Yonhap news agency reported, citing unnamed military officials.

The front-line island was declared a special security area, which could pave the way for the evacuation of about 300 remaining residents, journalists and officials.

Meanwhile, further south, a nuclear-powered U.S. supercarrier and a South Korean destroyer carried out naval drills in the Yellow Sea in a united show of force.

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."

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."