South Korea on Wednesday issued raid alerts for the Ulleungdo island, advising residents to evacuate to the nearest underground shelter, hours after North Korea fired 23 missiles, reported the Associated Press.

The warning was issued as one of the short-range ballistic missiles crossed the Northern Limit Line, the de facto maritime border between the two countries. The South Korean military said that the missile landed in waters just 57 kilometers east of the mainland.

The military said it was the first time since the peninsula was divided at the end of Korean War hostilities in 1953 that a North Korean missile had landed so close to its territorial waters.

It also claimed that 100 artillery rounds were fired by North Korea into a “buffer zone” near the maritime border between the two countries, reported the Associated Press.

The South Korean military added that its warplanes fired three air-to-ground missiles into the sea across the Northern Limit Line, reported Reuters. The military described the missile launch near South Korean territorial waters as “very rare and intolerable”.

In a statement, South Korean President Yoon Suk-yeol said that the provocation was “an effective territorial invasion”.

Yoon has called a National Security Council meeting and ordered “swift and stern measures so that North Korea’s provocations pay a clear price”. Japan’s Prime Minister Fumio Kishida told reporters he was planning a national security meeting as soon as possible.

Tensions have increased in the Korean peninsula in recent months as Pyongyang has tested a series of nuclear-capable missiles and also adopted a law allowing the preemptive use of the weapons on a broad number of issues, according to the Associated Press.

North Korea has argued its weapons tests are meant to be a warning to the United States and South Korea joint military drills that it sees as an invasion rehearsal. Last week, the joint military exercises involved about 240 warplanes.

In a statement released early on Wednesday, Pak Jong Chon, a secretary of the ruling Workers’ Party in North Korea, described the air force drills as “aggressive and provocative”.

Pak also accused Washington of planning the collapse of the North Korean regime, citing a report released by the Pentagon. The National Defense Strategy report had said that any nuclear attack by North Korea against the US or its allies “will result in the end of that regime”.