North Korea tests ballistic missiles in response to US-South Korea-Japan military drill

North Korea on Monday fired two ballistic missiles but one possibly blew up during an irregular flight, the South Korea military said, just a day after Pynongyang vowed to respond to joint military drills carried out by the US, Japan and South Korea.

The missiles were launched about 10 minutes apart in a northeasterly direction from the town of Jangyon in the country’s southeast, the South Korean military said.

The first missile flew about 600km and the second 120km, the military claimed, but didn't say where they landed.

North Korea typically fires missiles towards its eastern waters but the second missile's flight distance was too short to reach the sea as it possibly suffered an abnormal initial stage flight and blew up, raining debris inland.

Its trajectory meant the second missile may have fallen in an area close to the North's capital, Pyongyang, but the South's military spokesperson, Lee Sung-joon, said the launches were still under investigation.

The South’s military condemned the launches and said it was ready to repel any provocations by the North in conjunction with its chief military ally, the US.

"We strongly condemn North Korea's missile launch as a provocation that seriously threatens peace and stability on the Korean peninsula," the military said, adding that it shared information on the missiles with the US and Japan.

"The South Korean military will maintain its capacity and posture to respond overwhelmingly to any provocation while closely monitoring North Korea's various activities under a strong South Korea-US joint defence posture."

The missile launches by the North apparently came in response to a US-Japan-South Korea multidomain exercise in the region.

In recent years, the three countries have been expanding their trilateral security partnership to better cope with the North’s evolving nuclear threats and China's increasing assertiveness in the region.

The "Freedom Edge" drill was meant to improve upon previous exercises with simultaneous air and naval drills geared towards improving joint ballistic-missile defence, anti-submarine warfare, surveillance, and other skills and capabilities.

North Korea denounced the drill, calling the trilateral partnership an Asian version of Nato.

Its foreign ministry said the drill openly destroyed the security environment on the Korean peninsula and contained a US intention to lay siege to China and exert pressure on Russia.

The statement said North Korea will "firmly defend the sovereignty, security and interests of the state and peace in the region through offensive and overwhelming countermeasures".

Monday’s was likely the second failed missile launch by the North in five days. South Korea's military said the North fired what appeared to be a hypersonic missile on Wednesday but it spiralled out of control and exploded.

The first missile fired on Monday appeared to be similar to the KN-23 ballistic missile, which is believed to have been used by Russia in Ukraine.

North Korea is suspected of supplying ballistic missiles and artillery shells to Russia. Both nations have denied this despite pledges of military cooperation and a recently signed deal that includes promises of mutual military support.