South Korea's military has postponed a live-fire exercise planned for Tuesday on an island shelled by North Korea last week.
South Korean forces on Yeonpyeong island initially alerted residents Monday by loudspeaker to shelter in bunkers for a live-fire exercise to be held the next day.
Hours later, however, another broadcast on the island said the drill had been delayed. The South Korean military said the initial announcement by troops on Yeonpyeong was a mistake.
Yeonpyeong is located near the disputed western maritime border of the two Koreas and is surrounded by waters the North claims as its own. North Korea shelled Yeonpyeong last Tuesday in response to a South Korean exercise involving artillery fire from the island into the disputed waters.
The North Korean attack killed two South Korean Marines and two civilians, and drew return artillery fire from South Korean forces. South Korean President Lee Myung-bak warned Pyongyang Monday that it will "pay a dear price" if it attacks again.
His government has boosted its military presence on Yeonpyeong by deploying additional artillery guns and rocket launchers.
U.S. and South Korean naval forces also held a second day of exercises Monday in nearby waters in the Yellow Sea in a show of strength to deter North Korean attacks.
The Obama administration said U.N. sanctions against North Korea should be enforced more strictly, in response to the artillery strike and Pyongyang's recent disclosure of a new nuclear facility in the country.
U.S. Ambassador to the United Nations Susan Rice said Monday that Washington expects the U.N. Security Council's North Korea sanctions committee to intensify efforts to tighten sanctions enforcement.
In a nationally televised address Monday, Lee said North Korean shells that hit the island Tuesday landed a few meters from a school that was holding classes. He expressed outrage at what he called the "ruthlessness" of a North Korean leadership that he said is indifferent to the lives of children.
China has responded to the crisis by calling for emergency talks involving the six nations trying to negotiate an end to North Korea's nuclear program.
South Korea, Japan and the United States have said they are considering the Chinese proposal.
But, the spokesman for the U.S. Embassy in Beijing, Rob Raines, said any six-party talks cannot be a substitute for action by North Korea to comply with its obligations. He said those include the 1953 armistice that ended the Korean War and a six-party deal to scrap the North Korean nuclear weapons program.
Some information for this report was provided by AFP.