Library image of North Koreans watching a missile launch in July (Picture: KCNA).
North Korea has fired at least two unidentified weapons towards the Sea of Japan, the South says.
Military officials in Seoul could not confirm whether they were rocket artillery or ballistic missiles.
The South's Joint Chiefs of Staff said the projectiles were fired from a region near the North Korean capital of Pyongyang.
It is also unknown how far the objects traveled.
Japan's Defence Ministry said it believed the North Korean weapons were ballistic, but they did not reach Japan's territorial waters or its exclusive economic zone.
The latest launch follows frustration over the slow progress of nuclear talks with the US, and demands the Trump administration eases pressure on Pyongyang.
Earlier this month, the North fired its first underwater-launched ballistic missile in three years.