"Our military detected at around 6:00 am (2100 GMT Tuesday) a ballistic missile launched by North Korea from the Wonsan area towards the East Sea," the Joint Chiefs of Staff said in a message to reporters, referring to a body of water also known as the Sea of Japan.

Japan's Defence Ministry said on social media that the suspected ballistic missile had "already fallen", without providing further details.

Joint exercises between South Korea and the United States lasting 11 days are due to start on Monday, a regular fixture of the summer calendar designed to steel them for defence against the nuclear-armed North.

Pyongyang bristles at the drills -- calling them rehearsals for an invasion -- and has in the past staged pointed test missile launches before and after the exercises.