"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."
The exchange of fire came 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.
He added that the Obama administration should undertake a thorough review of North Korea, since the last one was in 2000.
ABC News' Joohee Cho, Jake Tapper, Kirit Radia, Luis Martinez and The Associated Press contributed to this report.