The North Korean merchant ship crossed the maritime boundary, known as the Northern Limit Line (NLL), northwest of Baengnyeong Island at around 5am (2000 GMT) on Friday, South Korea's Joint Chiefs of Staff said.

"Our forces issued warning broadcasts and fired warning shots, after which the vessel retreated beyond our waters," the JCS said in a statement.

The military added that they responded in "accordance" with procedures while "closely monitoring North Korea's movements".

North Korea does not formally recognise the NLL.

The incident comes two days after South Korea conducted live firing drills at the NLL.

The two Koreas technically remain at war because the 1950-53 Korean War ended in an armistice, not a peace treaty.

South Korea's President Lee Jae Myung, who took office in June, has vowed a more dovish approach towards Pyongyang compared with his hawkish predecessor Yoon Suk Yeol.

Lee vowed Tuesday at the United Nations to work to end the "vicious cycle" of tensions with the North as he promised not to seek regime change.