
North and South Korea have fired missiles into each other's coastal waters for the first time.
Seoul retaliated on Wednesday three hours after Pyongyang fired a missile that landed less than 60 km from the southern city of Sokcho.
The South's army said this was an "unacceptable" violation of its territory.
It fired three air-to-surface missiles in response, which officials said landed at a similar distance across the Northern Limit Line (NLL).
The demarcation line marks the approximate midway point in the sea between North and South Korea, but the North has never acknowledged the border, the BBC writes.
North Korea fired at least 10 missiles in the east and west on Wednesday, South Korean officials said.
It comes a day after the North threatened the US and South Korea over their joint military exercises, saying that "the US and South Korea will have to face a terrible case and pay the most terrible price in history ".
There is concern that North Korean leader Kim Jong-un may be preparing the ground for the first nuclear weapons test in five years.
At least one of North Korea's missiles, launched before 09:00 (00:00 GMT) on Wednesday, landed about 26 km south of the border, 57 km east of Sokcho and 167 km northwest of Ulleung Island.
It set off air raid sirens in Ulleung, where residents were told to evacuate to underground shelters.
He was immediately seized by South Korean and Japanese authorities, who quickly condemned the escalation by Pyongyang.
South Korean President Yoon Suk-yeol called it an "effective territorial invasion" and vowed a "swift and decisive response".
Under international law, countries can only make territorial claims for 12 nautical miles bordering their land, and the missile fell outside that boundary.
About three hours later, the South fired three precision air-to-surface missiles from warplanes into the waters off its eastern coast.
The shootings are a marked escalation of hostilities across the peninsula this year, which had already seen more than 50 missile launches from North Korea, including a ballistic missile that flew over Japan.