Washington pledged to defend South Korea after North Korean forces shot artillery shells at a southern island on Tuesday, prompting grave concern in capitals around the world.
In a powerfully-worded statement, the White House said the United States "strongly condemns this attack and calls on North Korea to halt its belligerent action."
It also urged nuclear-armed North Korea to "fully abide by the terms of the Armistice Agreement" that ended the Korean War of 1950-53.
"The United States is firmly committed to the defence of our ally, the Republic of Korea, and to the maintenance of regional peace and stability," it said.
Pyongyang's attack on the island near the Korean border killed at least two South Korean marines and Seoul's armed forces went on their highest state of alert in one of the most serious incidents since the Korean War.
US ally Japan, as well as China and Russia, also voiced their deep concern over the incident.
Japanese Prime Minister Naoto Kan, whose country has long had difficult relations with the reclusive communist state, ordered his government to prepare for any eventuality.
"I ordered (ministers) to make preparations so that we can react firmly, should any unexpected event occur," Kan told reporters after an emergency meeting of cabinet members and senior officials at his official residence.
"I ordered them to do their utmost to gather information."
Tuesday's incident came after nuclear-armed North Korea disclosed an apparently operational uranium enrichment programme -- a second potential way of building an atomic bomb -- causing serious alarm for the US and its allies.
A long-running, but currently stalled, six-nation negotiation process hosted by China and including both Koreas, the United States, Japan and Russia, is seeking to shut down the North's nuclear weapons programme.
China, the reclusive communist regime's only major ally, expressed concern over the artillery incident.
"We have taken note of the relevant report and we express concern over the situation," foreign ministry spokesman Hong Lei told reporters.
"We hope the relevant parties do more to contribute to peace and stability on the Korean peninsula."
Hong said it was "imperative" the six-nation talks were restarted "as soon as possible".
"It is China's consistent and firm position to realise de-nuclearisation on the (Korean) peninsula through dialogue and consultation," Hong said.
North Korea abandoned the forum in April 2009, a month before its second nuclear test, and announced in September last year it had reached the final stage of enriching uranium.
Russia also warned against an escalation of tensions.
"It's important that this does not lead to an aggravation of the situation on the Korean peninsula," an unnamed foreign ministry official told Interfax.