People in Guam were shocked to be given an emergency warning over the radio that the Pacific islanders were at risk.
The US territory is currently on edge after North Korea threatened to fire a missile into waters nearby.
Two radio stations activated an alert using the Emergency Alert Broadcast System, about “a civil danger”.
The warning, which was issued at 12.25am, was later confirmed to have been a mistake.
Several concerned listeners had called police over the alarm.
The alert prompted Guam’s homeland security adviser George Charfauros to urge residents and visitors to “remain calm”.
He said: “There is no change in threat level, we continue business as usual.”
The North Korean regime said on Friday, it was “carefully examining” a plan to strike Guam with missiles.
A spokesman for the country’s military said the plan could be “put into practice in a multi-current and consecutive way any moment”.
The threat came hours after President Donald Trump told Pyongyang that any threat to the US would be met with “fire and fury”.
In a new development, the North’s leader Kim Jong-un is said to have delayed a decision on striking Guam as he continues to observe US behaviour, opening up the possibility of a diplomatic solution to the crisis.
Guam’s Homeland Security Department confirmed in a statement that the “unauthorised test was not connected to any emergency, threat or warning”.
It said it would be working with the radio stations “to ensure the human error will not occur again”.
Overnight on Monday, images were release showing Mr Kim looking at plans to fire missiles into waters near US military bases on Guam.