The North Korean army, for its part, said a South Korean army ship had "entered" North Korean territory over a distance of 2.5 to five kilometers across the border. The North Korean army therefore fired ten artillery shells from the west coast as a warning.

The sea border was drawn by a US-led UN force at the end of the Korean War in 1953. Pyongyang has never recognized this border. At the sea border, there have been repeated incidents between the two Korean states over the years.

Tensions in the region have increased significantly in recent weeks, partly because of a series of North Korean missile tests. North Korea's army also fired hundreds of artillery shells and launched fighter planes in the border area with South Korea.

South Korea and the US have been warning for months that Pyongyang could prepare a nuclear weapons test. It would be the first such test since 2017.