South Korea and the United States fired eight missiles off the east coast of South Korea early Monday morning local time, in response to North Korea's ballistic missile test yesterday.
"We fired one missile into the Sea of Japan during a joint exercise with South Korea," Reuters quoted the US military as saying in a statement.
The statement added that the South Korean military launched seven missiles into the Sea of Japan during the exercise.
North Korea had launched 8 short-range ballistic missiles towards the sea off its eastern coast, after South Korea and the United States concluded their first joint exercises involving a US aircraft carrier.
South Korea's Joint Chiefs of Staff said the North fired the missiles from at least 4 different locations within 35 minutes, accusing North Korea of provocation.
According to reports, the missiles flew 110 to 670 kilometers before falling into the Sea of Japan, also known to Koreans as the East Sea.
The short-range ballistic missile has a range of about 1,000 km.
South Korean President Yoon Seok-yol said Pyongyang's nuclear and missile program constituted a "threat to world peace."
In turn, Japanese Prime Minister Fumio Kishida said that what North Korea is doing cannot be tolerated, adding that Tokyo will continue to work closely with the United States and South Korea to collect information and monitor alerts to ensure peace and security.
UN resolutions ban North Korea from testing ballistic missiles, which can carry nuclear warheads.
Sunday's launch is the 18th test of North Korean missiles this year, and comes one day after the US and South Korean Navy concluded a three-day maneuver in the Philippine Sea, in which the US aircraft carrier USS Ronald Reagan participated.