Air raid sirens have sounded in South Korea after the North fired about a dozen missiles in its direction, at least one of them landing near the rivals' tense sea border.
According to international media reports, Wednesday's launches came hours after North Korea threatened to use nuclear weapons to get the US and South Korea to "pay the most horrible price in history" as it has intensified its fiery rhetoric targeting the ongoing large-scale military drills between its rivals.
South Korea's military said North Korea launched more than 10 missiles of various kinds off its eastern and western coasts.
"The North Korean missile launch is very unusual and unacceptable as it fell close to South Korean territorial waters south of the Northern Limit Line for the first time" since the peninsula was divided, Kang Shin-chul, director of operations for the Joint Chiefs of Staff (JCS), told reporters.
An air raid warning was issued for the island of Ulleungdo, JCS said, which was flashed on national TV and told residents to "evacuate to the nearest underground shelter".
"We declare that our military will respond decisively to this," he added.
South Korea's military immediately fired three precision air-to-ground missiles near the spot on the maritime border where a North Korean ballistic missile landed earlier in the day.
The missiles were fired into waters "near the Northern Limit Line at a distance corresponding to the area where the North Korean missile struck," it said in a statement, adding the exercise showed Seoul would respond "sternly to any provocations".