The United States has pledged a "measured and unified" response to a North Korean artillery attack that rained shells on a South Korean island, killing two South Korean soldiers, dpa reported.
US President Barack Obama attended a Tuesday evening meeting of his senior national security team in Washington on the Korean situation, where he repeated the "unshakeable support of the United States for our ally," the White House said in a statement.
North Korea fired the artillery shells at a South Korean island near their disputed western sea border. South Korea returned fire for about one hour, and both sides warned of massive retaliation if there were further attacks.
Obama was awakened at about 4 am Tuesday with news of the attack, and was expected to call South Korean President Lee Myung-bak early Wednesday to reaffirm the US commitment to Seoul's defence.
"The president is outraged by these actions," White House spokesman Bill Burton said. "We stand shoulder to shoulder with our ally in South Korea."
Australian Prime Minister Kevin Rudd called the shelling "outrageously provocative" and urged China to bring Pyongyang to heel.
"I believe it's important now for China to bring all of its influence to bear on North Korea," he said.
Stephen Bosworth, US special representative for North Korea, arrived Tuesday in Beijing for talks with Chinese officials.
State Department spokesman Mark Toner called the attack by North Korea an "unprovoked military assault."
He emphasized that the Obama administration planned a "measured and unified approach" that would involve coordination with China and other nations in the six-party talks on North Korea's nuclear programme in a "deliberate, slow approach."
He vowed that Pyongyang would not be rewarded for belligerence.
"North Korea's behavior has been very, very bad; provocative and belligerent," Toner said.
The White House called for North Korea to abide by the ceasefire that ended the 1950-53 Korean War.
US stocks fell sharply amid the exchange of fire on the Korean Peninsula.
About 1,000 South Korean soldiers are stationed on Yeonpyeong, which has been a source of tension between the two neighbours because of its location and rich fishing grounds, and naval clashes have occurred nearby in 1999, 2002 and November 2009.
A South Korean warship was sunk in March near the border, killing 46 sailors. Seoul blamed Pyongyang for the sinking, but North Korea denied involvement.
Concerns have escalated recently over Pyongyang's nuclear programme, particularly after news over the weekend that it had a large, new uranium enrichment facility. North Korea said it is also building a light-water reactor.
In addition to the two South Korean soldiers who were killed, at least 13 were wounded, and four civilians were hurt by the shells.
China, one of North Korea's few allies, said it was troubled by reports of the exchange of fire, while noting: "The real situation needs to be confirmed."
"We hope relevant parties will do things conductive to peace and stability on the Korean peninsula," Foreign Ministry spokesman Hong Lei said in Beijing.
He called on the six-nation talks aimed at ending North Korea's nuclear programme to quickly resume. The negotiations - which involve China, the United States, North and South Korea, Japan and Russia - have been stalled since the end of 2008.