"The United States is firmly committed to the defense of our ally, the Republic of Korea, and to the maintenance of regional peace and stability," said Robert Gibbs, the White House press secretary, in a statement. "We are in close and continuing contact with our Korean allies."
The US called on North Korea to halt its belligerent action and to fully abide by the terms of the Armistice Agreement, the 1953 pact that ended the Korean War.
According to news reports, North Korea fired more than 100 artillery shells into Yeonpyeong Island off the west coast, killing two South Korean marines and wounding 21 people.
After the attacks began, according to Joongang Daily, Seoul dispatched F-16 fighter jets to the area that reported "suspicious movements" of military equipment on the North Korean coast near South Korea's Baekryeong Island, which is 17 km away from the North Korean coast.
On Tuesday evening, South Korea released a statement saying that it would "severely punish additional North Korean provocation" although officials repeated they are committed to making "the best efforts not to worsen the situation".
US inter-agency team on North Korea led by ambassador Steve Bosworth is in Tokyo and will be in Beijing on Wednesday. "The United States's position remains clear. North Korea has to take affirmative steps to denuclearise," US state department spokesman Philip J Crowley told reporters.
