According to information from Seoul, North Korea carried out weapons tests again shortly before the end of the year.
South Korea's military said on Saturday that North Korea fired three short-range ballistic missiles into the Sea of Japan around 8 a.m. local time (00 GMT).
The rockets were launched from Chunghwa district in northern Hwanghae province and flew about 350 kilometers, it said.
"Our military maintains an attitude of total readiness," South Korea's general staff said.
The isolated North Korea is constantly expanding its arsenal of weapons and also its nuclear capacities, the communist leadership in Pyongyang repeatedly threatens a military escalation in the region.
On Monday, several North Korean drones entered South Korea's airspace in the border regions around Gyeonggi province - one of them was flying near the capital Seoul.
UN resolutions ban North Korea from testing ballistic missiles of any range.
Depending on the design, such rockets can also be equipped with a nuclear warhead.
This year, North Korea has conducted an unprecedented number of weapons tests, including the November launch of its most advanced ICBM.
North Korea's ruler Kim Jong-un said earlier this year that he wanted to make his country the "most powerful nuclear power in the world".
The two Korean states have not signed a peace treaty since the Korean War (1950-53).