Launch comes as navies of South Korea, US and Japan stage trilateral anti-submarine exercises for the first time in five years.
North Korea has fired what appeared to be two ballistic missiles, South Korea and Japan said, in what is Pyongyang's fourth launch in a week.
At least two projectiles were launched from North Korea, the Japanese coast guard said early on Saturday, while Japan's public broadcaster NHK said the objects appear to have fallen outside Japan's exclusive economic zone.
"What appears to be a ballistic missile was launched from North Korea," the coast guard said in a statement issued at 6:47 am [local time].
In a second statement issued about 15 minutes later, the coast guard said another apparent ballistic missile was launched.
Pyongyang seeks to 'normailise' its tests
Analysts see the increased pace of testing as an effort to build operational weapons, as well as to take advantage of a world distracted by the Ukraine conflict and other crises to "normalise" its tests.
"Despite North Korea's internal weaknesses and international isolation, it is rapidly modernising weapons and taking advantage of a world divided by US-China rivalry and Russia’s annexation of more Ukrainian territory," said Leif-Eric Easley, a professor at Ewha University in Seoul.
Nuclear-armed North Korea has carried out a record-breaking blitz of weapons tests this year, including two ballistic missiles launched on Thursday, just hours after US Vice President Kamala Harris left South Korea.
South Korean and US officials have also been warning for months that North Korean leader Kim Jong-un is preparing to conduct another nuclear test.
North Korea, which is under multiple UN sanctions for its weapons programmes, typically seeks to maximise the geopolitical impact of its tests with careful timing.