Seoul's Joint Chief of Staff said it had detected multiple "short-range ballistic missiles launched into the East Sea around 07:10 from Pyongyang today (2210 GMT on Wednesday)," referring to the body of water also known as the Sea of Japan.

It said it was analysing details of the launch and "closely sharing information on the North Korean ballistic missile with the U.S. and Japanese authorities, while strengthening surveillance and vigilance in preparation for further launches".

Japan's defence ministry also confirmed the launch of at least one suspected North Korean ballistic missile, with the coastguard warning vessels to take care.

It is Pyongyang's first apparent weapons test since July 1, and comes days after the isolated, nuclear-armed country marked a key anniversary celebrating the founding of the ruling regime.

North Korea has regularly launched missiles around September 9, its foundation day, including conducting its fifth nuclear test on the same day in 2016.

The country's sixth nuclear test was conducted on September 3 in 2017.

At a speech to mark the 76th anniversary of its founding this week, leader Kim Jong Un said the country was moving to steadily increase its nuclear arsenal.

North Korea "will steadily strengthen its nuclear force capable of fully coping with any threatening acts imposed by its nuclear-armed rival states", Kim said.

- Trash balloons -

Relations between North and South Korea are at one of their lowest points in years, with the North recently announcing the deployment of 250 ballistic missile launchers to its southern border.

The North has also been bombarding the South with trash-carrying balloons, including a five-day straight blitz last week.

North Korea has sent more than 5,000 trash-filled balloons south since May, saying they are retaliation for propaganda balloons launched northwards by South Korean activists.

In response, Seoul has suspended a tension-reducing military deal with Pyongyang and restarted some propaganda broadcasts from loudspeakers along the border.

North Korea has recently bolstered military ties with Moscow, and analysts have said the North could be testing and ramping up production of artillery and cruise missiles before sending them to Russia for use in Ukraine.

The United States and Seoul have accused North Korea of supplying ammunition and missiles for Russia's war effort, a claim Pyongyang has called "absurd".

Earlier this year, Pyongyang declared the South its "principal enemy".

It has shuttered agencies dedicated to reunification and threatened war over "even 0.001 mm" of territorial infringement.

The North's latest ballistic missile launch comes weeks after South Korea and the United States wrapped their annual Ulchi Freedom Shield joint military drills.

North Korea -- which attacked its neighbour in 1950, triggering the Korean War -- has always been infuriated by joint US-South Korean military exercises, decrying them as rehearsals for invasion.