A US aircraft carrier is heading for the Korean peninsula a day after North and South Korea exchanged artillery fire on a border island, which left two South Korean soldiers and two civilians dead.
The USS George Washington, which carries 75 warplanes and has a crew of over 6,000, left a naval base south of Tokyo on Wednesday morning to join exercises with South Korea from Sunday, US officials in Seoul said.
"This exercise is defensive in nature," US Forces Korea said in a statement. "While planned well before yesterday's unprovoked artillery attack, it demonstrates ... our commitment to regional stability."
Earlier, Barack Obama, the US president, and Lee Myung-Bak, his South Korean counterpart, agreed "to continue the close security cooperation between our two countries".
A White House statement said Obama assured Lee that - with 28,000 troops based in South Korea - the US "stands shoulder to shoulder" with its ally.
Tuesday's incident on the Yellow Sea island of Yeonpyeong is one of the most serious since the Korean War ended in 1953.
The South is deploying more K-9 self-propelled guns, as well as replacing 105-mm howitzers with longer range weapons and bolstering a tank contingent.
Yonhap news agency, quoting South Korean defence ministry sources, said artillery on the North's southwestern coast were maintaining a ready-to-fire posture on Wednesday.
It also gave more details of North Korean military activity from Tuesday.
South Korea was conducting military drills in the area at the time but said it had not been firing at the North. The North maintains that the South started the fight.
North Korean MiG-23 jet fighters took off from Bukchang base for reconnaissance near the border before the shelling began, and they were later redeployed further south to Hwangju airbase, the source said.
The North had also deployed ground-to-ship missiles and put their warships in battle position, according to the report.
War of words
The South warned North Korea on Wednesday of "enormous retaliation" if it took more aggressive steps.
And North Korea said that South Korea was worsening ties on the peninsula with "reckless military provocation".
The North's official KCNA news agency said South Korea was "derailing the process for improving inter-Korean relations, scuttling inter-Korean Red Cross talks and driving the situation to the brink of war".
"We have seen this sort of provocation in the past from the North Koreans, although not at this intensity," Al Jazeera's Wayne Hay, reporting from the South Korean port town of Incheon, said. "This has taken things to a new level."
Outside the defence ministry in Seoul, some 110 protesters rallied, shouting "Vengeance!" They also burned North Korean flags and torched portraits of Kim Jong-il and his son, heir apparent Jong-Un.
Meanwhile, Chinese state media coverage of the shelling incident avoided criticising Pyongyang on Wednesday and even said the episode showed North Korea's "toughness".
China Central Television gave prominent play for a North Korean news broadcast railing at South Korea for starting the incident and threatening retaliation.
"North Korea showed its toughness during the skirmish," the Global Times newspaper said in an editorial that also criticised the "failure of the hard-line policies" of the current South Korean government towards its northern neighbour.
"The risk of escalation is very high," Paul Chamberlin, a Korea expert, told Al Jazeera. "The question is whether North Korea has enough wisdom to know what line not to go over."
North Korea's strikes aimed "to brandish heir apparent Kim Jong-Un's military prowess, strengthen internal unity and vent internal discontent toward the outside", Kim Hwang-Sik, the South Korean prime minister, told the National Assembly.
The South Korean premier said the shelling was a "premeditated, meticulously planned provocation".
The artillery attack comes two months after Kim Jong-Un, the youngest son of the current leader, consolidated his role by becoming a four-star general and vice chairman of the ruling party's Central Military Commission.
Defence Minister Kim Tae-Young, after talks with US counterpart Robert Gates, agreed that the two allies should step up their joint defence posture, including with US surveillance, in order to prevent further provocative acts by the North.
The attacks came after the North sparked fresh fears this month over its nuclear programme by showing a US scientist a modern uranium enrichment plant in Yongbyon outside the capital Pyongyang.