North Korea fired more than 20 missiles on Wednesday, including one that landed close to South Korea's waters for the first time since the peninsula was divided, South Korea's military said.
Air raid sirens sounded on a South Korean island and residents were evacuated to underground shelters.
In response, the South fired three air-to-ground missiles north of the maritime border between North and South Korea after President Yoon Suk-yeol called the incident “effectively a territorial invasion”.
South Korea said at least 23 missiles — 17 in the morning and six in the afternoon — were launched off its eastern and western coasts on Wednesday. It said the weapons were all short-range ballistic missiles or suspected surface-to-air missiles.
One of North Korea's short range ballistic missile crossed the Northern Limit Line, the maritime border between the two countries, prompting a rare warning for residents on the island of Ulleungdo to seek shelter in bunkers.
The 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” in protest at the South Korean-US military drills that it views as an invasion rehearsal.
The military said it was the first time since the peninsula was divided at the end of the of Korean War hostilities in 1953 that a North Korean missile had landed so close to the South's territorial waters.
“[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,” his office said.
The missile that was closest to South Korea landed in waters just 57 kilometres east of the South Korean mainland, the military said.
It said the launch near South Korean territorial waters was “very rare and intolerable”.
“Our military vowed to respond firmly to this (provocation),” it said.
An Air raid warning was issued for Ulleungdo that was flashed on national television and told residents to “evacuate to the nearest underground shelter”.
The South Korean military's Joint Chiefs of Staff initially said it detected the launch of three short range ballistic missiles.
But it later announced North Korea had fired “at least 10 missiles of various types today towards the east and west”.
Mr Yoon called a meeting of the National Security Council over the launches, ordering “swift and stern measures so that North Korea's provocations pay a clear price.”
Japan also confirmed North Korean missile launches, with prime minister Fumio Kishida saying he planned to call a “national security meeting as soon as possible”.
Pyongyang's latest test firing came as Seoul and Washington staged their largest-ever joint air drills, “Vigilant Storm” involving hundreds of war planes.
Pak Jong Chon, a high-ranking official in North Korea, said the drills were aggressive and provocative, state media reported.
Mr Pak said the name of the South Korea-US exercises harks back to Operation Desert Storm, the US-led military assault on Iraq in 1990-91 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.”
Wednesday's launches appeared to be “the most aggressive and threatening armed demonstration against the South since 2010,” Cheong Seong-chang, a researcher at the Sejong Institute, told AFP.
“It is now a dangerous and unstable situation that could lead to armed conflicts,” he said.
In March 2010, a North Korean submarine torpedoed the South Korean naval vessel Cheonan, killing 46 sailors, including 16 who were on their mandatory military service.
In November that year, the North shelled a South Korean border island, killing two marines, both of them young conscripts.
Wednesday's missile tests follow a recent blitz of launches, including what the North said were tactical nuclear drills.
Washington and Seoul have repeatedly said that the launches could culminate in another nuclear test, which would be Pyongyang's seventh.
“As far as I can remember, North Korea has never made such a provocation when South Korea and the US were holding their joint drills,” said Park Won-gon, a professor at Ewha University.
“Pyongyang seems to have completed its most powerful deterrent. This is a serious threat. The North also seem confident in their nuclear capabilities.”
