North Korea fired two short-range ballistic missiles into its eastern sea early Wednesday in what appeared to be a statement of defiance as the United States deploys a nuclear-armed submarine to South Korea for the first time in decades.

South Korea's Joint Chiefs of Staff said that from 3.30 to 3.46 am North Korea fired two short-range ballistic missiles from an area near capital Pyongyang that flew about 550 kilometers before landing in waters east of the Korean Peninsula.

Those flight details were similar to the assessment of the Japanese military, which said the missiles landed outside of Japan's exclusive economic zone and that there were no immediate reports of damage from ships or aircraft in affected areas.

The flight distance of the North Korean missiles roughly matched the distance between Pyongyang and the South Korean port city of Busan, where the USS Kentucky arrived Tuesday afternoon in the first visit by a US nuclear-armed submarine to South Korea since the 1980s.

Japanese Defence Minister Yasukazu Hamada told reporters that the North Korean missiles travelled on a low trajectory, with their maximum altitude reaching about 50 kilometers, and possibly demonstrated "irregular maneuver" in flight.

Japan has previously used similar language to describe the flight characteristics of a North Korean weapon modelled after Russia's Iskander missile, which travels at low altitudes and is designed to be maneuverable in flight to improve its chances of evading missile defences.

Wednesday's launches marked the North's first ballistic activity since July 12, when it flight-tested a new solid-fuel intercontinental ballistic missile that demonstrated potential range to reach deep into the US mainland. That launch was supervised by the country's authoritarian leader Kim Jong Un, who vowed to further bolster his country's nuclear fighting capabilities in the face of expanding US-South Korean military activities, which he blamed for worsening the security environment on the Korean Peninsula.

Tensions have rose in the region in recent months as the pace of both North Korean weapons tests and US-South Korean joint military drills have increased in a cycle of tit-for-tat.

The launches came as the US-led United Nations Command tries to secure the release of a US soldier who fled to North Korea from the South Korean side of a border village Tuesday afternoon.

Private 2nd Class Travis King, in his early 20s, had just been released from a South Korean prison where he was held on assault charges. Instead of getting on a plane to be taken back to Fort Bliss, Texas, he left and joined a tour of the Korean border village of Panmunjom, where he ran across the border, US officials say.

Since the start of 2022, North Korea has test-fired around 100 missiles while attempting to demonstrate a dual ability to conduct nuclear attacks on both South Korea and the continental United States. The allies in response have stepped up their joint military training and agreed to increase the deployments of US strategic assets like long-range bombers, air craft carriers and submarines to the region.

Periodic visits by US nuclear ballistic missile-capable submarines to South Korea were one of several agreements reached by the two countries' presidents in April in response to North Korea's expanding nuclear threat. They also agreed to establish a bilateral Nuclear Consultative Group and expand military exercises.