South Korea's military says it fired warning shots at a North Korean ship that crossed the de facto inter-Korean maritime border early on Monday. Pyongyang fired back in response.

South Korea's Joint Chiefs of Staff say a North Korean merchant vessel crossed the border about 27 kilometers northwest of the island of Baengnyeong in the Yellow Sea at around 3:42 a.m. local time.

The South Korean military says the vessel left the area following the warning shots.

Meanwhile, North Korea's General Staff of the Korean People's Army says a South Korean naval ship crossed the border, and that North Korean forces fired 10 shells from multiple rocket launchers toward the territorial waters at around 5:15 a.m.

North Korea has not recognized the de facto inter-Korean maritime border established by the US-led United Nations Command.

South Korea's Joint Chiefs of Staff have denounced the North for undermining the peace and stability of the Korean Peninsula and the international community.