North Korea fired a short-range projectile over the Sea of Japan on Tuesday, Yonhap News Agency reported, citing South Korean military sources.
The projectile travelled around 200 kilometres before striking a land target, Yonhap said.
"We are closely monitoring related developments and maintain a high level of readiness," an unnamed South Korean military official said, according to the report.
Tensions have been high on the peninsula since the beginning of the year, when Pyongyang conducted a nuclear test and long-range rocket launch, prompting increased international sanctions.
Most recently, North Korea fired five short-range missiles into the sea on March 21.