Troops at a US military base in South Korea heard an emergency siren instead of a bugle tune on Thursday, sparking fears of a North Korean attack.
A military spokesman blamed "human error" at Camp Casey, the closest US base to the border with the North.
North Korea had warned it could send a "Christmas gift" to the US to force concessions over stalled nuclear talks.
US President Donald Trump has refused to lift sanctions over the country's nuclear programme.
Separately, early on Friday Japan's public broadcaster NHK mistakenly reported that North Korea had fired a missile that landed in the sea off the Japanese coast.
NHK showed the newsflash on its website just after midnight local time but corrected the error 20 minutes later and apologised, saying the newsflash had been for "training purposes".
What happened at Camp Casey?
The wrongly broadcast siren reportedly "riled up" troops at the base and some were seen running in full uniform, according to an online post cited by the Washington Post newspaper.
A video purporting to have been filmed at the time was uploaded on to a Twitter account popular with US soldiers.
Spokesman Lt Col Martyn Crighton said he could not verify whether the video was of Thursday's incident.
Taps, a bugle call played at the end of the day on US bases, should have been played over the announcement system instead, he said.
Soldiers were quickly notified of the mistake, Lt Col Crighton said.
The strategically important base is near the South Korean town of Dongducheon about 60km (40 miles) north of the South Korean capital Seoul. It covers more than 10 sq km and is home to several thousand US troops.
Why are Korean peninsula tensions rising?
Earlier in December, North Korean Deputy Foreign Minister Ri Thae Song hinted that Pyongyang could resume long-range missile tests if Washington refused to change its negotiating position.
It was "entirely up to the US what Christmas gift it will select", he said at the time.
The US special envoy for North Korea Stephen Biegun has dismissed Pyongyang's deadline but said Washington was ready to resume talks at any time.
North Korea has also carried out tests at a satellite launch site to boost its nuclear deterrent, according to state media reports.
Last month, Japan condemned Pyongyang for "repeated launches of ballistic missiles" after two projectiles were fired.
The North however said it was testing a "super-large multiple-rocket launcher", and threatened that Japan "may see what a real ballistic missile is in the not distant future".
US President Donald Trump has said he still hopes to reach an agreement with North Korea.
The president made pursuing diplomacy with North Korea a centre-piece of his foreign policy agenda in 2018 but has failed to extract significant concessions on denuclearisation despite holding two summits with leader Kim Jong-un and even briefly setting foot in North Korea.