Fires were burning out of control and spreading after a South Korean island was hit by dozens of artillery shells fired by North Korea on Tuesday, a witness said.
YTN television quoted a witness as saying that many of the North Korean shells had landed at a military base on the island.
South Korea returned fire and scrambled F-16 fighter jets, military officials said. At least one South Korean marine reportedly was killed.
The conflict comes as military talks were about to take place between the Koreas, China and the US concerning the contested Northern Limit Line.