The launch from North Korea's eastern coastal Wonsan area was detected about 5pm, South Korea's Joint Chiefs of Staff said on Thursday, explaining that South Korea's military has bolstered its surveillance posture and is closely co-ordinating with the US and Japan.
Japan's defence ministry also said North Korea fired a possible ballistic missile but there was no impact in or around Japan, where the coast guard urged ships around the coast to use caution and notify authorities if any fallen objects were found.
The ballistic missile launch was North Korea's first since late June, when it tested a tactical ballistic missile with a "special mission" warhead and other weapons.
After watching the testing activities, leader Kim Jong-un underscored the need to bolster the military's "deadly and destructive offensive posture".
Thursday's launch also came before large-scale, annual exercises to be conducted by the US and South Korean militaries later this month.
US and South Korean officials say the training is defensive in nature, but North Korea says the manoeuvres are a practice invasion and often responds with its own missile tests and warlike rhetoric.
South Korean and US officials have not formally released details about 2026's joint exercise, but the drills are typically held in the latter half of August.
Kim has been focusing on enlarging its nuclear and missile arsenals since his high-stakes diplomacy with US President Donald Trump collapsed in 2019.
Experts say Kim wants international recognition as a nuclear state so that he can demand the lifting of international economic sanctions on North Korea.
UN Security Council resolutions ban North Korea from launches involving its huge stockpile of ballistic missiles.
Tests of other weapons including cruise missiles and artillery systems are not banned, but experts say they still pose a threat to South Korea and the US.
In early July, Kim supervised tests of a nuclear-capable cruise missile aboard a new 5000-tonne destroyer.
After years of prioritising ballistic missile development, Kim has increasingly turned his focus to naval capabilities, including the construction of a nuclear-powered submarine.
In recent years, Kim has boosted his diplomatic credentials by expanding ties with Russia.
North Korea has supplied conventional weapons and troops to Russia to support its war against Ukraine.
Kim also has pushed to deepen co-operation with China, the North's economic pipeline.
