Human error has been blamed for the accidental alarm at Camp Casey.
The operator immediately identified a mistake had been made and informed all units that it was a false alarm, Lieutenant Colonel Martyn Crighton, a public affairs officer, told the Associated Press news agency.
He added that no operations had been affected as a result of the incident on Thursday evening.
It came just a day before Japanese broadcaster NHK caused panic by mistakenly posting a news alert saying North Korea had fired a missile over Japan which had landed in the sea off the island of Hokkaido.
The broadcaster apologised, saying the alert was for media training purposes.
Both incidents come at a time of heightened tension in the region after North Korea set an end-of-year deadline for the US to offer mutually acceptable terms for a stalling nuclear deal between the two countries.
Kim Jong-un had previously said he was planning a “Christmas gift” for Donald Trump – widely interpreted to mean he was plotting new ballistic missile tests over the festive period – although Mr Trump himself appeared sanguine about the threat.
“Maybe it's a nice present,” he said at an armed forces rally. “Maybe it's a present where he sends me a beautiful vase as opposed to a missile test. I may get a vase. I may get a nice present from him. You never know."
Pyongyang has largely held off such tests since three meetings between the two leaders including a brief one at the border of North and South Korea in June this year.
However the North Korean tyrant is thought to be becoming increasingly frustrated at the impasse which has occurred since and the fact that crippling economic sanctions have remained in place.