South Korea's Joint Chief of Staff also said it had detected a "projectile" being fired towards what Seoul calls the East Sea.

Japan's coast guard, citing the defence ministry, said it had detected two ballistic missiles and that both were estimated to have already splashed down.

Japanese news agency Jiji Press reported the two missiles had landed outside of the country's Exclusive Economic Zone, citing defence ministry sources.

The test is Pyongyang's second of the month, following a salvo of missiles fired hours before South Korea's leader headed to China for a summit.

It comes a day after a high-level visit to Seoul by the Pentagon's number three official Elbridge Colby, who hailed South Korea as a "model ally".

North Korea has stepped up missile testing significantly in recent years.

Analysts say this drive is aimed at improving precision strike capabilities, challenging the United States as well as South Korea, and testing weapons before potentially exporting them to Russia.

Pyongyang is also set to hold a landmark congress of its ruling party in the coming weeks, its first in five years.

Ahead of that conclave, leader Kim Jong Un ordered the "expansion" and modernisation of the country's missile production.