South Korea's military says the North launched two projectiles that appear to be short-range ballistic missiles.
The Joint Chiefs of Staff said in a text message to reporters that the projectiles were fired eastward from Sunan airport in Pyeongyang City.
The military said it is closely monitoring the situation, to be ready for any additional launches by the North.
The defense ministry said South Korea and U.S. intelligence agencies are analyzing the latest move.
This is the fourth launch this month, taking place just three days after the North fired the same type of missiles last Friday.
The North reported that they were ballistic missiles launched from a train, marking the North's second known launch using a railway platform, following the first one last September.
Seoul's military said its initial assessment showed that last Friday's missiles flew about 430 kilometers, at a maximum altitude of 36 kilometers.
The North also launched missiles last Tuesday and on January 5th, and claimed that both of those launches were successful tests of its "hypersonic" missiles.
Hypersonic missiles are seen as a game-changer because they are easier to maneuver and operate at high speeds meaning they are harder to track and intercept.
Last Tuesday's launch was conducted under the watch of its leader Kim Jong-un.
Pyeongyang is banned from developing or testing ballistic missiles and nuclear weapons under multiple UN Security Council resolutions.
Bae Eun-ji, Arirang News.