South Korea and the United States jointly fired eight missiles on Monday, in response to a volley of ballistic missiles fired by North Korea a day earlier.
It comes as Pyongyang continues to escalate its missile tests.
South Korea’s president Yoon Suk-yeol said his government would respond sternly to any provocation from its northern neighbour.
“We will make sure there isn’t a single crack in protecting the lives and property of our people,” he said.
Speaking at a war memorial event in Seoul, he added that North Korea’s nuclear and missile programmes “are reaching the level of threatening not only peace on the Korean Peninsula but also in Northeast Asia and the world”, according to Yonhap.
The US and South Korea regularly hold joint military exercises, which often anger North Korea.
Early on Monday, hours after North Korea had fired several missiles off its east coast, the two allies launched eight surface-to-surface Army Tactical Missile Systems (ATACMS).
Such displays had been rare under South Korea’s previous administration.
President Yoon, who was inaugurated last month, has pledged to take a more hardline approach on North Korea.
The UN prohibits North Korea from ballistic and nuclear weapons tests, and has imposed strict sanctions after previous tests.