North Korea and South Korea exchanged fire on Monday after shells landed in the south's territory following drills in the north, BBC News reported.
The north's military faxed a message about the fire drills to the south's navy on Monday. In response, the south said they would fire back if anything landed in their territory.
"Some of [North Korea's] shells landed south of the border during the drill. So our military fired back north of the border in line with ordinary protocol," a defense ministry statement said.
According to Defense Ministry spokesman Kim Min-seok, the north "fired some 500 shots... and some 100 of them landed in waters south of the border."
In return, the south fired 300 shots, he said.
In addition, residents of Baengnyeong -- an island in the border -- were evacuted during the three-hour incident.
Though the United Nations drew the border between the north and south after the Korean War, the north has never officially recognized it.
In late 2010, two Marines and two civilians in South Korea were killed by North Korean artillery fire, which the north tried to justify by claiming they were responding to military exercises in the south.
A year later, fire was briefly traded on the border as well.
