North Korea fired an unspecified ballistic missile toward the East Sea, known as the Sea of Japan, on Tuesday, according to a report citing the South Korean military.
The move came a day after South Korea and the US began "a regular military exercise," Yonhap News Agency reported.
The agency reported that the launch came after the North fired two "strategic cruise missiles" from a submarine Sunday and conducted a "fire assault drill" three days earlier.
Seoul and Washington began the Freedom Shield exercises which are scheduled to run for at least 10 days from Monday.