The US and South Korea launched eight ballistic missiles into the sea in response to North Korean provocation.
As part of the exercise, eight Army Tactical Missile System missiles — one American and seven South Korean — were fired into South Korea’s eastern waters across 10 minutes.
The tit-for-tat missile launches aim to demonstrate the South’s ability to respond swiftly and accurately to North Korean attacks, the South Korean military said.
It comes after North Korean leader Kim Jong-un ordered eight short-range ballistic missiles to be launched on Sunday in the country’s largest single test, local reports said.
South Korea’s Joint Chiefs of Staff said at least eight missiles were fired from the Sunan area of the North Korean capital Pyongyang and they flew between 110km and 600km at altitudes between 25km to 90km.
The missiles landed in the sea off the east coast.
South Korea convened a National Security Council (NSC) meeting where President Yoon Sukyeol ordered "expanded deterrence of South Korea and the United States and continued reinforcement of united defence posture" following the missile launches.
The NSC meeting concluded the missile launch was North Korea’s "test and challenge" of the security readiness of South Korea’s new administration.
Kim Gunn, its Special Representative for Korean Peninsula Peace and Security Affairs, discussed the "provocation" with US officials.
North Korean state media have yet to comment on Sunday’s launches.
They came after the US aircraft carrier Ronald Reagan concluded a three-day naval drill with South Korea in the Philippine Sea on Saturday.
North Korea has long condemned the allies’ combined military exercises as invasion rehearsals and often countered with its own missile drills, including short-range launches in 2016 and 2017 that simulated nuclear attacks on South Korean ports and US military facilities in Japan.