The US Strategic Command (USSTRATCOM) said the launch, detected just after midday yesterday Korea time, was believed to be of a much-hyped Musudan missile which North Korea has now test-launched seven times, with only one partial success.
After a string of five failed launches, North Korea test fired a Musudan in June that flew 400 kilometres into the Sea of Japan (East Sea).
Such launches are usually detected within hours or even minutes by the South Korean and US militaries, and it was unclear why Sunday's USSTRATCOM announcement came so long after the event.
There was no immediate confirmation from the South Korean side.
US weapons analysts say successful Musudan testing could help the nuclear-armed North develop an operational intercontinental ballistic missile (ICBM) capable of striking the US mainland by 2020, a US think-tank said Friday.US Defence Secretary Jim Mattis has said that President Donald Trump and his military team are aware of the missile launch. "The President and his military team are aware of North Korea's most recent unsuccessful missile launch. The President has no further comment," said Mattis.