North and South Korea have both fired missiles across their maritime border for the first time.
The South retaliated on Wednesday three hours after Pyongyang fired a missile which landed less than 60km (37mi) from its eastern city of Sokcho.
Seoul said such a firing 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.
The South said the North fired at least 10 missiles on Wednesday. These are believed to be in response to joint military drills being carried out by the US and South Korea this week.
At least one of the missiles- launched just before 09:00 (00:00 GMT) on Wednesday – landed about 26km south of the border, 57km east of Sokcho and 167km north-west of Ulleung island.
The launch had triggered air-raid sirens on Ulleung, and residents were told to evacuate to underground shelters.
It was picked up immediately by South Korean and Japanese authorities who swiftly condemned the escalation from Pyongyang.
South Korea’s President Yoon Suk-yeol labelled it an “effective territorial invasion” and vowed a “swift and firm response”. However, countries technically only have a right to establish its territorial sea to a limit not exceeding 12 nautical miles (22km).
Around three hours later, the South fired three precision air-to-ground missiles from warplanes in waters off its east coast.
