Seoul on Wednesday issued an air raid alert after North Korea fired short-range missiles that landed near South Korean waters for the first time.
The Yonhap News Agency in South Korea quoted an official on the island saying that employees took shelter in a basement when the air raid warning for Ulleung was broadcast on national television.
President Yoon Suk-yeol convened a meeting of the National Security Council, and condemned the "unprecedented" launches, which took place amid a period of national mourning for the 156 people killed in the Itaewon crowd crush at the weekend.
Sending a missile south of the NLL was "tantamount to territorial intrusion," he was quoted as saying in a statement following the NSC meeting.
The missile launch by North Korea is followed by Pyongyang's warning over ongoing military exercises between South Korea and the United States, reported Al Jazeera.
Denouncing North Korea's launch of its first missile into an area close to the South's territorial waters since the peninsula's division more than seven decades ago, South Korea's military called it an "intolerable" act, reported Yonhap News Agency.
The missile, one of at least 10 missiles fired on Wednesday morning, was detected by South Korea's military and the Japanese coast guard, and landed less than 60km (37 miles) off South Korea's coast.
The missile was one of a number of short-range missiles launched from the area around Wonsan on North Korea's eastern coast at about 8.51 am (23:51 GMT), according to South Korea's Joint Chiefs of Staff (JCS) and an air raid warning for the island of Ulleung was issued shortly afterwards, reported Al Jazeera.
The JCS said at least one of the missiles landed 26km (16 miles) south of the Northern Limit Line, which serves as an unofficial maritime border between the two Koreas; the first time that has happened since the peninsula was divided.
The launch was "very unusual and absolutely unacceptable," the JCS said, adding that it would respond "decisively".
The missile landed 57km (35 miles) from the South Korean city of Sokcho on the east coast, and 167km (104 miles) from Ulleung, reported Al Jazeera.
North Korea has carried out an unprecedented number of weapons tests this year - now more than double the launch record set in 2019 - and there are expectations it may soon resume nuclear testing.
Japan said it had detected two suspected ballistic missiles launched from the North, one flying east and the other southeast.
"North Korea has repeatedly been launching missiles at an unprecedented rate, in new ways that we have not seen before," Defence Minister Yasukazu Hamada told reporters in Tokyo on Wednesday.
"These actions threaten the peace and stability of Japan, the wider region, as well as the broader international community, and are utterly unacceptable."
Japan has lodged a protest against the launches via diplomatic channels in Beijing, he added.
Pyongyang argues its weapons are necessary for 'self-defence' and that recent tests are a warning to Washington and Seoul over joint military drills that it views as an invasion rehearsal, reported Al Jazeera.
This week's exercises, dubbed Vigilant Storm, began on Monday and are some of the two allies' largest, involving about 240 warplanes from both sides staging mock attacks 24 hours a day.
The White House says such drills are part of the US's routine training schedule with South Korea.
"We reject the notion that they serve as any sort of provocation. We have made clear that we have no hostile intent towards the DPRK and call on them to engage in serious and sustained diplomacy," White House National Security Council Spokesperson Adrienne Watson said on Tuesday, using North Korea's official name, the Democratic People's Republic of Korea.
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