Tensions have risen even more between North and South Korea with the discovery of two dead South Korean civilians from the North's bombing of a tiny island,
The burned bodies of two men, believed to be in their 60s, were found amid the rubble of a destroyed construction site. It was previously announced that two South Korean marines were killed in the attack and 18 other people injured. Casualty numbers from the North are not available.
On Tuesday the North bombed the island of Yeonpyeong after the South refused to stop military exercises near the disputed area. The South responded with artillery fire of its own in a skirmish that lasted about an hour. Secretary-General Ban Ki-moon called one of the "gravest incidents" since the Korean War.
As South Korean troops remained on high alert, President Obama reiterated Washington's pledge to "stand shoulder to shoulder" with the South, and called on China, one of the North's only allies, to restrain its friends.
China said it was "highly concerned" about the exchange and urged restraint.
China "feels pain and regret about an incident causing deaths and property losses and is worried about the developments," Foreign Ministry spokesman Hong Lei said in a statement. "We have always maintained that the relevant parties should, through dialogue and consultation, resolve disputes by peaceful means."
While there have been several outbreaks of aggression since a cease fire was declared more than 50 years ago (the Korean War has never technically ended), this is the first strike against a civilian population. About 30 homes were destroyed. Experts say South Koreans see the killing of civilians as taking the confrontation to a new level.
"It's clearly a line for people, and crossing that line puts it in a different category," said John Delury, an assistant professor at Seoul's Yonsei University Graduate School of International Studies. "People here are feeling very conflicted, outrage and sorrow."