North Korea has fired what has been described as an unidentified projectile into the sea, said South Korea's Joint Chiefs of Staff (JCS).
The Japanese coast guard, which first reported the launch, said it could potentially be a ballistic missile, but no confirmation has yet been given, BBC reports.
The UN prohibits North Korea from ballistic and nuclear weapons tests.
If confirmed, this would be the first such launch carried by Pyongyang this year.
"South Korean and US intelligence are closely analysing for further detail," the JCS said in a statement.
Japan's defence minister Nobuo Kishi said the suspected ballistic missile had flown about 500 km (310 miles), according to a Reuters report.
The launch comes days after Kim said that Pyongyang would continue to strengthen its defence capabilities due to an increasingly unstable military environment on the Korean peninsula.
He made the remarks during a key end-of-year meeting of North Korea's ruling party.
Japanese Prime Minister Fumio Kishida called the latest launch "very regrettable," pointing to North Korea's repeated testing of missiles since 2021.
In 2021, North Korea continued the advancement of its weapons programme, conducting what state media reported as the testing of a new hypersonic missile, as well as a train-based ballistic missile and a new long-range cruise missile.
Ballistic missiles are considered more threatening than cruise missiles because they can carry more powerful payloads, have a longer range and can fly faster.
