South Korea has fired back missiles towards North Korea after the rogue nation launched the most missiles in one day, including one that crossed the disputed maritime border for the first time.

South Korean President Yoon Suk-yeol said the missile that came within 60km of the coastal town of Sokcho.was effectively a “territorial invasion”.

It was the first time a ballistic missile had landed near the South’s waters, prompting rare air raid warnings that sent residents rushing to bunkers.

Pyongyang launched at least 23 missiles — the most in one day — including the one that landed off South Korea’s coast, crossing the Northern Limit Line (NLL) sea border.

Seoul responded with warplanes firing three air-to-ground missiles towards the north over the disputed maritime demarcation line as President Yoon vowed a “swift and firm response”.

The tit-for-tat launches near each other’s coasts and over the maritime border was a marked escalation in hostilities between the neighbouring nations.

South Korea said the closest missile had landed “near our territorial sea” for the first time since the Korean peninsula was divided after the Korean War (1950-1953).

However under international law, countries can only claim to 12 nautical miles off their coastline.

News broadcasts in South Korea warn residents of Ulleung island to seek shelter. Photo: Getty

A local official on the island of Ulleung described the panic that the North Korean missiles caused.

“We heard the siren at around 8:55 am and all of us in the building went down to the evacuation place in the basement,” the official told Reuters.

“We stayed there until we came upstairs at around 9:15 after hearing that the projectile fell into the high seas.”

North Korea this week threatened “the most horrible price in history” if the South continued its joint military drills with the USA that began on Monday.

The North also fired more than 100 rounds of artillery from its east coast into a military buffer zone established in a military agreement with the South, South Korea’s military said.

Nuclear-armed North Korea has tested a record number of missiles this year, and officials in Seoul and Washington say the North has completed technical preparations to conduct a nuclear weapon test for the first time since 2017.

The launches came just hours after Pyongyang demanded that the United States and South Korea stop large-scale military exercises.”

The US and South Korea began one of their largest combined military air drills on Monday.

Dubbed Vigilant Storm, the exercises involve hundreds of warplanes from both sides staging mock attacks 24 hours a day.

North Korea had said that a recent flurry of launches were in response to allied drills.

Pak Jong Chon, secretary of the Central Committee of North Korea’s ruling Workers’ Party, said in a statement on Wednesday that the number of warplanes involved in Vigilant Storm proved the exercise was “aggressive and provocative” and specifically targeted North Korea.

He said even its name imitated the US-led Operation Desert Storm against Iraq in the 1990s.

On Tuesday US State Department spokesperson Ned Price said that the drills were “purely defensive in nature” and that the United States had made clear to North Korea that it harboured no hostile intent towards the country.

Mr Price added that the US and its allies had also made clear that there would be “profound costs and profound consequences” if North Korea resumed nuclear testing, which would be a “dangerous, destabilising step”. He did not elaborate on the consequences.

In a phone call with U.S. Secretary of State Antony Blinken, South Korean Foreign Minister Park Jin called the North Korean missile launches a “grave act of military provocation”.

Japan defence minister Yasukazu Hamada said North Korea’s actions threaten the peace and stability of Japan, the wider region, as well as the broader international community, and are utterly unacceptable.

-with AAP