Its military activities are always a source of tension.
But an exercise on Monday ordered by North Korea’s Supreme Leader Kim Jong-un has ended in an exchange of artillery fire with South Korea.
The South says it returned fire after North Korean shells crossed the disputed western maritime border and landed in its territorial waters. South Korean islanders living nearby fled to bomb shelters.
Pyongyang had notified Seoul that drills were imminent and warned shipping to avoid the area.
The border, drawn up at the end of the Korean War, is not recognised by the North and there have been deadly clashes in the past.
A South Korean navy ship was sunk four years ago near the area of the latest infringement. An international team of investigators said it was torpedoed by the North, but Pyongyang denied the charge.
Months later the North bombarded a village on a South Korean island in the same area, killing four people.
Amid annual, joint military exercises between South Korea and American forces, Pyongyang has been flexing its military muscle in recent weeks to demonstrate its anger.
The North claims the drills are a rehearsal for an invasion.