North Korea fired at least 23 missiles on Wednesday. It is extremely rare for the country to launch more than 20 missiles in a single day.

The South Korean military said the North fired at least 17 short-range ballistic and other missiles from several sites in the morning. It added that the North also launched six surface-to-air and other missiles in the afternoon. The military said those missiles flew toward the Sea of Japan and the Yellow Sea.

The military said one of the three short-range ballistic missiles launched in or around Wonsan in the east between 8 a.m. and 9 a.m. crossed a maritime boundary called the Northern Limit Line or NLL, which was set by UN forces. It added the missile fell in international waters 167 kilometers northwest of Ulleungdo Island.

South Korea's Joint Chiefs of Staff denounced North Korea, saying it is the first time that a North Korean missile has crossed the NLL and fallen near the country's territorial waters since the division of the two Koreas.

An air raid alert was issued for the island. South Korean media report that it is the first such alert in the country in six years and nine months.

In a rare move, South Korean fighter jets fired three air-to-surface missiles toward international waters on the northern side of the NLL.

The South Korean military said the North fired about 100 artillery shells toward the Sea of Japan in response.

North Korea's missile launches come as South Korea and the United States are jointly conducting large-scale air force drills.