"North Korea's ICBM launch presumed to have ended in failure," Seoul's military said, adding that it was "assessed to have failed in second stage separation".

Seoul's Joint Chiefs of Staff said they had detected the launch of a long-range ballistic missile -- the ICBM -- early Thursday, followed shortly by two short-range ballistic missiles.

"The range of the long-range ballistic missile is around 760 kilometres, altitude of 1,920 kilometres at speed of Mach 15," the military said.

"The short-range ballistic missiles flew around 330 kilometres at an altitude of around 70 kilometres at speed of Mach 5.

"North Korea's successive launch of ballistic missiles is a serious provocation that harms the peace and stability of not only the Korean Peninsula but also the international community," the military said.

"The South Korean military will maintain a firm readiness posture to overwhelmingly respond to any provocation from North Korea."

The launches come during Seoul and Washington's largest-ever joint air drills, known as Vigilant Storm, involving hundreds of warplanes from both sides. They will end Friday.

Such exercises infuriate Pyongyang, which sees them as a rehearsal for invasion and said Wednesday that South Korea and the United States would "pay the most horrible price in history" if they failed to halt

North Korea fired more than 20 missiles on Wednesday, including one that landed near South Korea's territorial waters, which Seoul said was "effectively a territorial invasion".

In March, North Korea is believed to have failed in an apparent attempt to test-fire Kim Jong Un's largest and most powerful ICBM, the Hwasong-17.

Seoul and Washington said at the time that the missile exploded shortly after launch, with the explosion visible in the sky above North Korean capital Pyongyang.

Another ICBM test is also believed to have failed in May, Seoul's military said at the time.