Pyongyang has fired three ballistic missiles into Sea of Japan, U.S. military said early Saturday.
President Moon Jae-in was immediately briefed over the incident, the JCS added.
Japan's Kyodo news service said that the projectile could be a ballistic missile.
The launch comes weeks after North Korea created a tense standoff with the United States by threatening to lob some of its missiles toward Guam.
North Korea also successfully flight-tested a pair of intercontinental ballistic missiles in July that analysts say could reach deep into the U.S. mainland when perfected.