North Korea fired more than 10 missiles on Wednesday, including one that landed close to South Korea's waters, reports AFP.
North Korea President Yoon Suk-yeol remarked it as "effectively a territorial invasion".
One short range ballistic missile crossed the Northern Limit Line, the de facto maritime border between the two countries, prompting a rare warning for residents on the island of Ulleungdo to seek shelter in bunkers.
The military said it was the "first time since the peninsula was divided" at the end of the Korean War hostilities in 1953 that a North Korean missile had landed so close to the South's territorial waters.
"President Yoo pointed out today that North Korea's provocation is an effective territorial invasion by a missile that crossed the Northern Limit Line for the first time since the division," his office said in a statement.
The missile that was closest to South Korea landed in waters just 57 kilometers (35 miles) east of the South Korean mainland, the military said.
The military released a statement describing the missile launch near South Korean territorial waters as "very rare and intolerable".
Bd-pratidin English/Lutful Hoque
