North Korea test fired as many as two rounds of short-range ballistic missiles Monday, South Korean officials said.
The missiles were fired east from the Soonan airport area of Pyongyang, according to South Korea's Joint Chiefs of Staff.
"Currently, South Korean military is surveilling related activities to the possible additional launch while maintaining readiness," it said.
The launch follows tests on Jan. 5, 11 and 14.
Experts believe the activity is reprisal for new sanctions the Biden administration imposed last week over the North's continued test launches.
Earlier in the week, North Korean leader Kim Jong Un called for strengthening the country's strategic military forces after observing the test of what state media said was a hypersonic missile. It was the first time since March 2020 that he had officially attended a missile test.
The Japanese Defense Ministry said the latest launch was detected shortly after 9 a.m. Monday.
"Vessels are advised to pay attention to further information and, if they see a falling object, to stay away from it and report the relevant information to the Japan Coast Guard," the coast guard said in a statement.