North Korea fires about a dozen missiles, including one that lands close to South Korea's territorial waters, prompting Seoul to issue a rare warning for people on an island and fire three missiles in response.
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.
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".
The JCS had initially said it detected the launch of three short-range ballistic missiles but later announced North Korea had fired "at least 10 missiles of various types today towards the east and west".
It said one of the missiles landed 26 kilometres away from the rivals' sea border. South Korea’s military said it was the first time a North Korean missile had landed very close to the sea border since the countries' division in 1948.
South Korean President Yoon Suk-yeol called a meeting of the National Security Council over the launch, ordering "swift and stern measures so that North Korea's provocations pay a clear price."
Yoon "pointed out today that North Korea's provocation is an effective territorial invasion by a missile that crossed the Northern Limit Line for the first time since the division" of the peninsula, his office said in a statement.
Japan also confirmed North Korean missile launches, with Prime Minister Fumio Kishida telling reporters he planned to call a "national security meeting as soon as possible."
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Pyongyang warns Seoul, Washington
Pyongyang's latest launch comes as Seoul and Washington stage their largest-ever joint air drills, dubbed "Vigilant Storm", which involve hundreds of warplanes from both sides.
Pak Jong Chon, a high-ranking official in North Korea, said the drills were aggressive and provocative, according to a report in state media on Wednesday.
Pak said the name of the exercises harks back to Operation Desert Storm, the US-led military assault on Iraq in 1990-1991 after it invaded Kuwait.
"If the US and South Korea attempt to use armed forces against the (Democratic People's Republic of Korea) without any fear, the special means of the DPRK's armed forces will carry out their strategic mission without delay," he said.
"The US and South Korea will have to face a terrible case and pay the most horrible price in history."
READ MORE: US and South Korea begin largest air drills ever