"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."

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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, the DPRK issued another threat to attack the ROK 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, the DPRK 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 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 the DPRK.

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 the DPRK'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.