(Lee Ji-eun ~ Associated Press)
Violence often erupts in this slice of sea claimed by both countries. Boats routinely jostle for position during crab-catching season, and three deadly naval clashes since 1999 have taken a few dozen lives.
The South's president took responsibility Monday for failing to protect his citizens from a deadly North Korean artillery barrage on Yeonpyeong Island on Nov. 23. The origins of the attack can be traced to a sea border drawn at the close of the Korean War, nearly 60 years ago.
As the conflict ended in a truce, the U.S.-led U.N. Command divided the Yellow Sea without Pyongyang's consent, cutting North Korea off from rich fishing waters and boxing in a crucial deep-water port, a move that clearly favored the South.
North Korea has bitterly contested the line ever since, arguing that it should run farther south. But for Seoul, accepting such a line would endanger fishing around five South Korean islands and hamper access to its port at Incheon.
"It is the perfect recipe for 'accidental' warfare," Erich Weingartner, editor-in-chief of CanKor, a Canadian website focused on North Korean analysis, wrote recently.
"The navies of both sides protect their respective fishing vessels. Mischief and miscalculation does the rest," he added. "The outbreak of hostilities is less surprising to me than the fact that for 60 years these hostilities have been contained."
The Nov. 23 attack hit civilian areas in Yeonpyeong, marking a new level of hostility along the contested line. Two civilians and two marines died, and many houses were gutted in the shelling.
Normally home to about 1,300 civilian residents, the island was declared a special security area Monday, which could pave the way for a forced evacuation of those who did not flee last week. Military trucks carrying what appeared to be multiple rocket launchers were seen heading to a marine base on the island.
Long-range artillery guns and a half-dozen K-9 howitzers were also on their way, the Yonhap news agency reported, citing unidentified military officials.
South Korean President Lee Myung-bak, in a nationally televised speech, vowed tough consequences for any future aggression, without offering specifics.
"I feel deeply responsible for failing to protect my people's lives and property," he said.
After his speech, Yeonpyeong officials announced new live-fire drills for today, warning residents to take shelter in underground bunkers. Another announcement later in the evening said there would be no exercise; marines on the island had failed to get final approval from higher authorities.
Last week's attack came on the same day South Korea conducted artillery drills from the island. The North says it warned Seoul that morning not to fire into the disputed waters.
The North's anger has only increased as a nuclear-powered U.S. aircraft carrier and a South Korean destroyer take part in previously scheduled joint military exercises this week farther south in the Yellow Sea.
Earlier today, the North's propaganda machine warned that the military drills could trigger a "full-blown war" on the peninsula.
"Our republic has a war deterrent that can annihilate any aggressor at once," the government-run Minju Joson said in a commentary carried by the official Korean Central News Agency.
The U.N. Security Council is separately studying how to respond to the shelling as well as recent revelations of what the North says is a new uranium enrichment plant, U.S. Ambassador Susan Rice said Monday.
China, North Korea's only major ally, has sought to calm tensions by calling 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 March sinking of a warship, reacted coolly to the proposal.
Japan rejected a new round of aid-for-disarmament talks any time soon. Foreign Ministry Seiji Maehara said in an interview with national broadcaster TBS late Monday that North Korea must first honor past commitments on disarmament.
On Sunday, North Korea described the disputed waters as "the most acute and sensitive area where military conflict might break out anytime due to the illegal 'northern limit line' unilaterally drawn by the U.S. and the ceaseless provocation of the South Korean puppet group."
The U.N. Command demarcated the line after failed attempts to negotiate a sea border. Andrei Lankov, a North Korea expert at Kookmin University in Seoul, said the move clearly favored the South, exploiting the weakness of the North Korean navy.
Many experts believe North Korea would be given greater territorial waters than it currently has if the issue were settled by arbitration or some other impartial means, said Daniel Pinkston, a Seoul-based analyst for the International Crisis Group.
"International law is on their side in this case, but it does not justify the action they've taken in any way," he said. "In fact, it undermines their legitimate arguments for the establishment of an equitable maritime boundary."
The waters were the scene of deadly skirmishes in 1999, 2002 and 2009 and then, in March, the worst attack on Seoul's military since the Korean War.
A South Korean-led international investigation found that a North Korean torpedo sank the Cheonan, a 1,200-ton South Korean warship, killing 46 sailors. The North denied it.
Rodger Baker, an analyst for the U.S. security think tank STRATFOR, said the North is stepping up its efforts to draw the world's attention to its push for a change in the maritime border.
"They're now shelling South Korean islands," he said. "The question is how far do the North Koreans have to go before the crisis either draws attention in the way they want or forces a response from the South Koreans and, ultimately, from the United States?"
Klug reported from Seoul, South Korea. AP writers Hyung-jin Kim in Seoul and Anita Snow in the United Nations contributed to this report.