It is not known yet whether the missiles were short, medium or long range, such as an intercontinental ballistic missile (ICBM).
The US tracks all North Korean weapons tests through radar and satellites.
US satellites are able to detect heat signatures of a missile launch almost immediately.
Earlier in the day, the South Korean Joint Chiefs told reporters that North Korea had launched an unidentified projectile toward the sea between the Korean peninsula and Japan.
The Japanese Coast Guard has warned ships to be on the lookout for falling debris.
One official said North Korea launched short-range projectiles, possibly artillery or cruise missiles, not ballistic missiles.