The launch on Tuesday would be the latest weapons test by North Korea, which has pressed ahead with military development in the face of international sanctions imposed over its nuclear weapons and missile programs.
The missile was launched from the vicinity of Sinpo, JCS said, where North Korea keeps its submarines as well as equipment for test firing submarine-launched ballistic missiles (SLBMs). North Korea has also launched other types of missiles from that area.
The launch came as the intelligence chiefs of the United States, South Korea, and Japan were due to meet in Seoul to discuss the standoff with North Korea, amid other issues, Yonhap news agency reported, citing a government source.
Representatives of hundreds of international companies and international militaries were gathered in Seoul for the opening ceremonies of the International Aerospace and Defense Exhibition (ADEX).
It is set to be South Korea's largest defense expo ever, with displays of next-generation fighter aircraft, attack helicopters, drones, and other advanced weapons, as well as space rockets and civilian aerospace designs.
South Korea is preparing to test fire its first homegrown space launch vehicle on Thursday.