North Korea has fired a missile towards the South, which crossed the two countries' maritime border for the first time since the peninsula's split.
The short-range ballistic missile landed some 60km (37 miles) from the South's city of Sokcho and triggered air-raid alarms on Ulleungdo island.
South Korea later fired three missiles in response to the North.
Seoul's President Yoon Suk-yeol had called Pyongyang's launch an "effective territorial invasion".
Pyongyang fired at least 10 missiles "east and west" on Wednesday morning, the South Korean military said.
Later on Wednesday, South Korea's military said it had fired three air-to-ground missiles towards north of its maritime border, in response to the North's launch.
It had earlier declared that military could not "tolerate this kind of North Korea's provocative act, and will strictly and firmly respond under close South Korea-U.S. cooperation," said the Joint Chiefs of Staff in a statement.
They added South Korean President Yoon Suk-yeol had ordered a "swift response" to the latest aggression.
Both South Korea and Japan's leaders have called national security meetings in response to North Korea's latest firings.
Both countries had recorded the missiles just before 09:00 (00:00 GMT) on Wednesday, including the one which breached the Northern Limit Line - the defacto maritime border.
That missile had landed in waters 26km (16 miles) south of the demarcation line, 57km east of the South Korean city of Sokcho and 167km north-west of Ulleungdo island.
This was "very unusual and unacceptable" as it had fallen close to "territorial waters south of the Northern Limit Line for the first time" since the peninsula was divided, said Kang Shin-chul, director of operations for the Joint Chiefs of Staff.
The missiles come a day after Pyongyang warned the US and South Korea to stop its joint military drills this week around the peninsula.
On Tuesday, North Korea had threatened that if the allies didn't stop their drills, it would take "powerful" measures.
North Korea's launches on Wednesday follows a blitz of missiles it fired last month which it said were also in response to US, South Korea and Japan joint drills. It had described its response as a "simulation" for a nuclear attack on the South.
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