The reclusive communist country has conducted six sanctions-busting launches in less than two weeks, the latest coming on Thursday with the firing of a pair of ballistic missiles.

On Tuesday, the North fired an intermediate-range ballistic missile over Japan, prompting a call for people in affected areas underneath to take cover.

"The missile test launch by the DPRK is a regular and planned self-defensive step for defending the country's security and the regional peace from the US direct military threats that have lasted for more than half a century," North Korea's civil aviation agency said without specifying which launch, according to state run news agency KCNA.

Seoul, Tokyo and Washington have ramped up joint military drills in recent weeks, and carried out more exercises Thursday involving a US Navy destroyer from the USS Ronald Reagan aircraft carrier's strike group.

The launches are part of a record year of weapons tests by isolated North Korea, which leader Kim Jong Un has declared an "irreversible" nuclear power, effectively ending the possibility of denuclearisation talks.

Analysts say Pyongyang has seized the opportunity of stalemate at the UN to conduct ever more provocative weapons tests.

Officials in Seoul and Washington have been warning for months that Pyongyang will also conduct another nuclear test, likely after China's Party Congress on October 16.