North Korea fired at least 17 missiles into the sea on Wednesday, including one that landed less than 60 km (40 miles) off South Korea's coast, which the South's President Yoon Suk-yeol described as "territorial encroachment" by Pyongyang.

It was the first time a ballistic missile had landed near the South's waters since the peninsula was divided, and the most missiles fired by the North in a single day. South Korea issued rare air raid warnings and launched its own missiles in response as tensions spiralled in the region.

The missile landed outside South Korea's territorial waters, but south of the Northern Limit Line (NLL), a disputed inter-Korean maritime border.

South Korean warplanes fired three air-to-ground missiles into the sea north across the NLL in response, the South's military said. An official said the weapons used included an AGM-84H/K SLAM-ER, which is a U.S.-made "stand-off" precision attack weapon that can fly for up to 270 km (170 miles) with a 360 kg (800-pound) warhead.

The South's launches came after Yoon's office vowed a "swift and firm response" so North Korea "pays the price for provocation".

"North Korea's provocation today was an effective act of territorial encroachment by a missile intruding the NLL for the first time since (the two Koreas') division," Yoon's office said.

The North Korean weapon was one of three short-range ballistic missiles fired from the North Korean coastal area of Wonsan into the sea, South Korea's Joint Chiefs of Staff (JCS) said. The JCS later said as many as 14 other missiles of various types had been fired from North Korea's east and west coasts.

The JCS said at least one of the missiles landed 26 kilometres south of the NLL, 57 kilometres (35 miles) from the South Korean city of Sokcho, on the east coast, and 167 kilometres from the island of Ulleung, where air raid warnings were issued.

"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," an Ulleung county official told Reuters. "We stayed there until we came upstairs at around 9:15 after hearing that the projectile fell into the high seas."

A resident on the southern part of the island said they received no warnings.

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.

The firing violates the 2018 agreement, the JCS 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.

North Korea continues to test ballistic missiles despite multiple United Nations Security Council resolutions that ban all ballistic and nuclear tests by the country
North Korea continues to test ballistic missiles despite multiple United Nations Security Council resolutions that ban all ballistic and nuclear tests by the country

The launches came just hours after Pyongyang demanded that the United States and South Korea stop large-scale military exercises, saying such "military rashness and provocation can be no longer tolerated."

Despite Yoon's declaring a national week of mourning after more than 150 people were killed in a weekend crowd surge in Seoul, the United States 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.