North Korea fired a short-range ballistic missile into the sea off its east coast on Friday, South Korea's military has said.
The country also flew fighter jets close to the border with South Korea and fired artillery shells, reports said.
The launch is the latest in a series by the nuclear-armed country, which the US warned could culminate in a nuclear test.
South Korea said it had detected the ballistic missile launch from the Sunan area in North Korea's capital Pyongyang early on Friday.
Earlier, the north flew 10 fighter jets close to the shared Korean border, crossing a “reconnaissance line”, which triggers a response.
Seoul then scrambled military aircraft, including F-35A fighter jets, the Joint Chiefs of Staff said.
North Korea also fired about 170 artillery shells into the waters off its east and west coasts, in breach of a maritime “buffer zone” deal agreed in 2018, the Joint Chiefs of Staff said.
Pyongyang has increased missile launches and military exercises in recent weeks, which it now describes as “tactical nuke” drills.
South Korea and the US say North Korean leader Kim Jong-un could soon conduct his country's seventh nuclear test.
North Korea missile tests — in pictures
South Korean President Yoon Suk-yeol said North Korea’s provocations were becoming “indiscriminative”.
He said his country had massive retaliation capabilities that could deter North Korean assaults to some extent.
“The decision to attack can’t be made without a willingness to risk a brutal outcome,” Mr Yoon told reporters.
“The massive punishment and retaliation strategy, which is the final step of our three-axis strategy, would be a considerable psychological and social deterrence [for the North].”
A rare military statement from North Korea said its actions were in response to a “provocative” South Korean artillery exercise near the border.
The Korean People's Army “took strong military countermeasures”, the official Korean Central News Agency reported on Friday.
The country issued “a stern warning to the South Korean military [for] inciting military tension in the frontline area with reckless action”, said the statement.
Earlier this week, Mr Kim dismissed the idea of restarting talks over North Korea's banned weapons programmes, saying his country “felt no necessity to do so”.