The United States has urged restraint following a North Korean artillery attack on South Korea and vowed to forge a "measured and unified" response with major powers, including China.
North Korea fired several dozen artillery shells at a South Korean island in one of the heaviest bombardments of the South since the Korean War ended in 1953, sharply increasing tensions on the divided peninsula.
South Korea warned North Korea of "enormous retaliation" if it took more aggressive steps. But the United States, which has about 28,000 troops stationed in South Korea, played down the chances of any immediate US military action to deter the reclusive state.
US Defence Secretary Robert Gates and South Korean Defence Minister Kim Tae-young agreed to co-ordinate any response to the North Korean action, the Pentagon said, adding that Gates expressed his appreciation for the "restraint shown to date."
The United States labelled the attack a violation of the armistice agreement which ended the 1950-53 Korean War, as it did North Korea's March attack on a South Korean warship that killed 46 South Korean sailors.
But State Department spokesman Mark Toner said the United States was seeking a unified diplomatic front with North Korea's neighbours, including China, Pyongyang's sole remaining major backer, which has in the past resisted international efforts to get tough with its isolated ally.
"We're going to take a measured and unified approach," said Toner, adding, "We're not going to respond willy nilly."
The White House condemned the attack. US President Barack Obama was awoken at 3:55am for an emergency briefing and was outraged over the strike, the White House said. He was due to speak with South Korean President Lee Myung-bak, White House spokesman Bill Burton said.
"North Korea has a pattern of doing things that are provocative. This is a particularly outrageous act," Burton said aboard Air Force One as Obama headed to Indiana to visit an auto plant.
Obama's top national security aides, including Secretary of State Hillary Clinton and Defence Secretary Robert Gates, met on Tuesday to discuss the situation, the White House said.
The South fired back after Tuesday's attack and sent fighter jets to the area, but no US forces were involved in the South's response, a US official said.
Global stock markets fell in reaction to the escalating tensions. In the United States, major stock indexes such as the Dow Jones Industrial Average fell about 1.5 percent, while an investor flight to safety pushed up gold and the US dollar
The artillery attack posed the second test in three days of Washington's vow that it will not reward what it deems bad behaviour with diplomatic gestures, like resuming aid-for-disarmament talks.
The attack followed revelations over the weekend of a uranium enrichment facility - a second source of atomic bomb material in Pyongyang's nuclear programme.
In Washington, Republican lawmakers took the lead in calling for a tougher approach to North Korea.
"Two decades worth of attempts to appease this North Korean regime have failed, and further attempts to do so will meet with the same result," said Senator John McCain, the Republican presidential nominee in 2008, calling on Beijing to "play a more direct and responsible role in changing North Korea's reckless behaviour."
Analysts said the North may be again pursuing a strategy of calculated provocations to wrest diplomatic and economic concessions from the international community.