The BBC's John Sudworth explains how the cross-border clash developed
South Korea says it has returned fire after North Korea fired dozens of artillery shells at one of its border islands, killing two marines.
The South's military was placed on its highest non-wartime alert after the shells landed on Yeonpyeong island.
Pyongyang accused the South of firing first. The Southern military said it had conducted exercises but shelling was directed away from the North.
This is one of the worst clashes since the Korean War ended, analysts say.
There have been occasional cross-border incidents since the conflict ended without a peace treaty in 1953, but the latest comes at a time of rising regional tension.
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