White House press secretary Robert Gibbs called on North Korea to "to halt its belligerent action and to fully abide by the terms of the armistice agreement," the 1953 pact that ended the Korean War.
North Korea fired barrages of artillery onto a South Korean island near their disputed western border , setting buildings ablaze and killing at least one marine after warning the South to halt military drills in the area.
Mr Gibbs said the White House "is in close and continuing contact" with the South Korean government.
"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," he said.
The attack came amid high tension over North Korea's claim that it has a new uranium enrichment facility and just six weeks after North Korean leader Kim Jong Il unveiled his youngest son Kim Jong Un as his heir apparent.
Yesterday the State Department said it was studying the evidence a group of visiting American scientists used to conclude the North was building the enrichment facility, which presumably could be used to produce fuel for nuclear weapons.
"We will not be drawn into rewarding North Korea for bad behaviour," he said. "They frequently anticipate doing something outrageous or provocative and forcing us to jump through hoops as a result. We're not going to buy into this cycle."
Earlier this month, during a speech to US troops stationed in South Korea, President Barack Obama said: "Pyongyang should not be mistaken: The United States will never waver in our commitment to the security of the Republic of Korea. We will not waver."