South Korea's Joint Chiefs of Staff said it had detected the launch at around 3:50 am (1850 GMT Wednesday) of "what North Korea claims is a space launch vehicle," it said in a statement.

The projectile was fired southwards from North Phyongan province and "traversed the international airspace above waters west of Ieodo," it said, referring to the Socotra Rock in the Yellow Sea.

"Our military is maintaining a full readiness posture and closely coordinated with the United States, while simultaneously elevating our security posture," the JCS added.

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 would be "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.

The launch also follows Seoul and Washington kicking off their major annual joint military drills on Monday.

Known as Ulchi Freedom Shield, the exercises, which are aimed at countering growing threats from the nuclear-armed North, will run through August 31.

Pyongyang views all such drills as rehearsals for an invasion and has repeatedly warned it would take "overwhelming" action in response.

Suspected North Korean hackers have already targeted the exercises.

- Second attempt -

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.

The crash sparked a complex, 36-day South Korean salvage operation involving a fleet of naval rescue ships, minesweepers and deep-sea divers.

The retrieved parts of the rocket and the satellite were analysed by experts in South Korea and the United States, who later said it had no military utility as a reconnaissance satellite.

North Korean leader Kim Jong Un has made the development of a military spy satellite a top priority.

North Korea's ruling party "bitterly" criticised the officials responsible for the crash in June, according to state media.

Relations between the two Koreas are at their lowest point in years, and diplomacy is stalled after failed attempts in recent years to discuss Pyongyang's denuclearisation.

Kim has declared North Korea an "irreversible" nuclear power and has called for ramped-up arms production, including tactical nuclear weapons.