North Korea bombarded a South Korean island near their disputed western border Tuesday, setting buildings ablaze and killing at least two marines after warning the South to halt military drills in the area, South Korean officials said.

South Korea said it returned fire and scrambled fighter jets in response, and said the "inhumane" attack on civilian areas violated the 1953 armistice halting the Korean War. The two sides technically remain at war because a peace treaty was never negotiated.

Hours after the skirmish, North Korea's supreme military command threatened to continue strikes against its rival if it violated their disputed sea border "even 0.001 millimeter," according to the North's official Korean Central News Agency.

The bombardment 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.

The United States, which has tens of thousands of troops stationed in South Korea, condemned the attack and called on North Korea to "halt its belligerent action," White House Press Secretary Robert Gibbs said in Washington. He said the United States is "firmly committed" to South Korea's defense, and to the "maintenance of regional peace and stability."

The North's artillery struck the small South Korean-held island of Yeonpyeong, which houses military installations and a small civilian population and which has been the focus of two previous deadly battles between the Koreas.

Two South Korean marines were killed and 16 injured, the Joint Chiefs of Staff said. Island residents escaped to about 20 shelters in the island and sporadic shelling ended after about an hour, according to the military.

The firing came during South Korean military drills in the area. North Korea's military had sent a message to South Korea's armed forces early Tuesday to demand that the drills stop, but the South continued them, the JCS said.