Gibbs said the White House "is in close and continuing contact" with the South Korean government.
"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," he said.
Congressional Republicans and Democrats joined the administration in condemning the attack.
"As the people of the Republic of Korea question what new belligerent action may come from the North, they should not have any question that the people and forces of the United States stand ready as a devoted ally committed to the defense of their nation," Senate Minority Leader Mitch McConnell, R-Ky., said in a statement. "I join the president in his strong condemnation of what is sadly just the latest in a long string of hostile actions. North Korea's neighbors should unite in condemning this attack."