Hawaii has tested a nuclear attack warning siren for the first time since the end of the Cold War.
The resumption of the monthly tests comes amid a growing threat from North Korea’s missile and nuclear program.
North Korea has tested a series of ballistic missiles and in September carried out its sixth nuclear test.
Hawaii already has a monthly test of sirens warning of natural disasters, including tsunamis.
The nuclear attack signal uses a different, wavering tone, warning residents and tourists to stay indoors and await further instructions.
The last time a nuclear attack warning siren was tested in the state was in the 1980s in the final years of the Cold War.
However, it sounded again on December 1 and will be repeated on the first business day of every month.
Vern Miyagi, the head of the Hawaii Emergency Management Agency, said it was “critically important” for the public to understand what the different tones mean, the Honolulu Star-Advertiser reported.
A missile launched from North Korea could strike Hawaii within 20 minutes of launch, the publication added.
Hawaii hosts the US military headquarters for the Asia-Pacific region.
Pyongyang recently tested a new intercontinental ballistic missile which it claims can hit anywhere on the mainland United States.
Experts say the Hwasong-15 appears capable of transporting a nuclear warhead, although it is unclear if North Korea is yet capable of making a weapon small enough to be fitted on to a missile.