North Korea fired off at least one apparent short-range ballistic missile into the Yellow Sea on Thursday night, Seoul said, just days ahead of large-scale joint military exercises between South Korea and the U.S.

The South Korean military said it had detected the launch from the North Korean port city of Nampo on its western coast at 6:20 p.m., adding later that it was also looking into the possibility that the North had fired multiple SRBMs simultaneously from the same area.

"While strengthening its monitoring and vigilance, our military is maintaining a full readiness posture in close cooperation with the United States," the South Korean military said, according to the Yonhap news agency.