South Korea’s military has said the missile was fired from an area north of Pyongyang early on Wednesday local time.
The Yonhap news agency reported South Korea’s joint chiefs of staff saying that it and US authorities are analysing the trajectory.
A US official also said a launch had taken place.
The launch is the first since September 15 when North Korea fired an intermediate ballistic missile.
US President Donald Trump was briefed on the apparent launch, the White House said.
Press secretary Sarah Sanders said in a tweet Mr Trump “was briefed, while missile was still in the air, on the situation in North Korea”.
At the time of the launch, Mr Trump was in a meeting with Senate Republicans on Capitol Hill.
Japan’s chief cabinet secretary said the missile might have landed inside the country’s exclusive economic zone in the Sea of Japan.
Yoshihide Suga said the missile appears to have been fired from North Korea’s western coast.
President Donald Trump, commenting on North Korea’s latest missile launch, has said: “We will take care of it.”