South Korea's president vowed on Thursday, to boost troops on an island targeted by a North Korean artillery barrage, while the North stridently warned of additional attacks if the South carries out any "reckless military provocations."
Seoul and Washington also increased pressure on China to use its influence on ally North Korea to ease soaring tensions that erupted after an exchange of fire Tuesday that left four South Koreans dead, including two civilians. China urged both sides to show restraint.
The North's bombardment of a tiny South Korean island of Yeongpyeong along a disputed maritime frontier alarmed world leaders including President Barack Obama, who reaffirmed plans for joint maneuvers with Seoul involving a nuclear-powered aircraft carrier in the Yellow Sea starting Sunday.
"We should not let our guard down in preparation for another possible North Korean provocation," South Korean President Lee Myung-bak said during an emergency meeting on security and economic repercussions from the attack, presidential spokesman Hong Sang-pyo said. "I think a similar North Korean provocation could come at any time."
South Korea will boost ground troops on Yeongpyeong and four other islands in western waters response to this week's attack, reversing a 2006 decision calling for an eventual decrease, Hong said.
He declined to discuss specific numbers for the increase, but said troops there currently amount to about 4,000. The attack spiked tensions that already were high due to March's sinking of a South Korean warship in the same tense waters that killed 46 sailors in the worst military attack on the nation since the Korean War.
The two Koreas are required to abide by an armistice signed at the close of the three-year war, but the North does not recognize the maritime line drawn by UN forces in 1953 and considers South Korean maneuvers near Yeonpyeong island a violation of its territorial waters.
Skirmishes occur from time to time around the sea border, but Tuesday's attack was the first to target civilians and raised concerns about a unprecedented escalation of tensions that could lead to another war.
The shelling also comes as North Korea is undergoing a delicate transition of power from leader Kim Jong Il to his young son Kim Jong Un.
The US - South Korean drills scheduled for Sunday and involving aircraft carrier USS George Washington in waters south of Yeongpyeong, although previously scheduled, are sure to infuriate North Korea.
The North made no specific mention of those exercises in its statement but warned that its military would "launch second and third strong physical retaliations without hesitation if South Korean warmongers carry out reckless military provocations."
The North's statement also said Washington was to blame for South Korean artillery exercises earlier in the week near disputed waters which prompted the North to respond with its artillery barrage on Yeongpyeong island Tuesday.
Washington "should thoroughly control South Korea," it said. The warning was issued by North Korea's military mission at the truce village of Panmunjom and was carried by the country's official Korean Central News Agency.
Meanwhile, the Obama administration has urged China to rein in its ally North Korea, with the chairman of the Joint Chiefs of Staff, Admiral Mike Mullen, saying, "We really think it's important for the international community to lead, but in particular China."
Chinese Premier Wen Jiabao called on all sides to show "maximum restraint" and urged the international community to work to ease tensions.
He repeated Chinese calls for renewed six-nation talks aimed at persuading North Korea to dismantle its nuclear programs. Wen said those talks, involving the two Koreas, China, Russia, Japan and the United States, are the best way to ensure stability on the peninsula and its denuclearization.
Wen's remarks were made in Russia on Wednesday on a state visit and posted on the Chinese Foreign Ministry's website. South Korea said it will increase diplomatic efforts to get China to put pressure on North Korea, according to Yonhap.
Repeated calls to the presidential office seeking confirmation went unanswered. Residents of Yeongpyeong who evacuated the island and began arriving at the South Korean port of Incheon on Wednesday told harrowing tales of fiery destruction and narrow escapes.
Ann Ahe-ja, one of hundreds of exhausted evacuees from Yeonpyeong island arriving in the port of Incheon on a rescue ship, said the artillery barrage that killed four people, two of them civilians, had caught her by surprise.
"Over my head, a pine tree was broken and burning," Ann told AP Television News on Wednesday. "So I thought 'Oh, this is not another exercise. It is a war.' I decided to run. And I did." About 10 homes suffered direct hits and 30 were destroyed in the midafternoon barrage, according to a local official who spoke by telephone from the island just seven miles (11 kilometers) from the North Korean shore. About 1,700 civilians live on Yeonpyeong alongside South Korean troops stationed there.
"I heard the sound of artillery, and I felt that something was flying over my head," said Lim Jung-eun, 36, who fled the island with three children, including a 9-month-old strapped to her back. "Then the mountain caught on fire."
Many of those evacuated from Yeonpyeong had spent the night in underground shelters and embraced tearful family members on arrival in Incheon.
The shower of artillery from North Korea was the first to strike a civilian population. In addition to the two marines killed, the bodies of two men, believed in their 60s, were pulled from a destroyed construction site, the coast guard said. At least 18 people, most of them troops, were injured.
Officials in Seoul said there could be considerable North Korean casualties. North Korea's main Rodong Sinmun newspaper published a military statement accusing South Korea of triggering the exchange, but did not mention any casualties.