North and South Korea have both fired missiles landing in waters off each other’s coasts for the first time, the BBC reports.
Seoul retaliated on Wednesday three hours after Pyongyang launched a missile that landed less than 60km off the South’s city of Sokcho.
The South’s military said this was an “unacceptable” breach of its territory.
It fired three air-to-ground missiles in response, which officials said landed a similar distance past the Northern Limit Line (NLL).
The demarcation line marks the rough midway-point in the sea between North and South Korea, but the North has never accepted the boundary.
On Tuesday North Korea warned that South Korea and the US would pay “the most horrible price in history” if they continued joint military drills, seen as a veiled threat to use nuclear weapons.
The North is expected to resume nuclear weapons testing soon after a five-year break, with US and South Korean intelligence saying Pyongyang has completed all the necessary preparations.