North Korea fired what appeared to be a ballistic missile towards the Sea of Japan on Tuesday, Tokyo's defense ministry said.
South Korea's Joint Chief of Staff also said it had detected a "projectile" being fired towards what Seoul calls the East Sea.
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, AFP said.
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 modernization of the country's missile production.
