"In a phone call this morning, Secretary Gates told Minister Kim the United States strongly condemns the attack by North Korea, views it as a violation of the armistice agreement and assured him that we are committed to South Korea's defense," Pentagon Press Secretary Geoff Morrell told reporters. "He expressed sympathy for the loss of life and appreciation for the restraint shown to date by the South Korean government."
President Lee Myung-bak convened a national ministers' security meeting to discuss countermeasures at his underground bunker office and instructed his secretaries to stem spread of the armed clash.
"(We) should carefully manage the situation to prevent the escalation of the clash," said Lee.
The western sea border has been at the center of dispute where the two Koreas fought bloody skirmishes in 1999, 2002, and most recently November 2009. But this is the first time since the end of the Korean war in 1953 that North Korea has fired on South Korea's civilian territory.
The exchange of fire comes days after North Korea revealed its upgraded and strengthened uranium enrichment plant to western scientists.
Siegfried Hecker, the Stanford professor and former head of the Los Alamos lab who was invited to North Korea last week to witness their new uranium program, said today the country's nuclear capability is much farther along than previously estimated.
The White House released a statement early this morning condemning the attack: "The United States strongly condemns this attack and calls on North Korea to halt its belligerent action and to fully abide by the terms of the Armistice Agreement. 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."
National Security Advisor Tom Donilon called President Barack Obama and woke him up at 3:55 a.m. to update him on the situation.
Stephen Bosworth, U.S. envoy on North Korea, raised concerns about North Korea's actions during his meetings with Chinese officials in Beijing today, but as of now, there has been no change in his travel plans. Bosworth, who was in Seoul and Tokyo earlier this week for consultations on North Korea's newly unveiled uranium facility, is slated to return to the Unites States tomorrow.
ABC News' Jake Tapper, Kirit Radia, Luis Martinez and the Associated Press contributed to this report.