North Korea fired several cruise missiles off its west coast into the water. The test is the third that Pyongyang has conducted in less than a week.
South Korea's military "detected several unknown cruise missiles launched into the West Sea of North Korea around 7 am [local time] (2200 GMT)," Seoul's Joint Chiefs of Staff said in a statement.
It added that South Korean and US intelligence agencies were "conducting a detailed analysis." The two countries' militaries were "closely monitoring North Korea's activities."
The chiefs of staff gave no details on the number of missiles fired, how far they flew or if they were launched from land or sea.
Intensified cruise missile testing
North Korea has intensified its cruise missile testing this year.
On Sunday, South Korea's Joint Chiefs of Staff (JCS) said they detected multiple cruise missiles over waters near the North Korean port of Sinpo, where the North has a major military shipyard building key naval vessels, including missile-firing submarines.
Earliear on Thursday, North Korea said it had carried out its first test of a new generation of "strategic" cruise missiles.
The country's official Korean Central News Agency said the Pulhwasal-3-31 missile is still in its development phase, adding that the launch did not pose a threat to neighboring countries.
"Strategic" typically refers to weapons that are nuclear capable.
Rising regional tensions
North Korean cruise missile activities, unlike their ballistic counterparts, aren't directly banned under current UN sanctions.
Since 2021, North Korea has conducted numerous tests of what it called long-range cruise missiles fired from both land and sea.
Tensions in the region have risen in recent months as North Korean leader Kim Jong Un continues to ramp up weapons development and make provocative threats.
In response, the United States, South Korea and Japan have been expanding their combined military exercises.