South Korea's Joint Chiefs of Staff said Pyongyang had fired "what the North claims is a space launch vehicle," without offering further details.

The launch was first signaled by the Japanese government, which said the North had fired a suspected ballistic missile. The Kyodo agency, citing the government in Tokyo, said the projectile had flown over Japanese territory.

Pyongyang told Japan's coast guard on Tuesday that its launch would take place between August 24 and 31, prompting Tokyo to mobilise ships and its PAC-3 missile defence system in case anything was to land in its territory.

Seoul has said such a launch is "an illegal act" because it violates UN sanctions prohibiting the North from tests using ballistic technology, which is used for both space launches and missiles.

The launch came days after leaders from Washington, Seoul and Tokyo met at Camp David in the United States, with North Korea's growing nuclear threats a key item on the agenda.

In May, Pyongyang launched what it described as its first military reconnaissance satellite, but the rocket carrying it, the "Chollima-1" -- named after a mythical horse that often features in official propaganda -- plunged into the sea minutes after takeoff.