Guam has raised its official threat level and on Thursday tested its emergency alert system after warnings from North Korea identifying the island as a potential missile target.
With a North Korean missile test expected at any time, authorities said the US territory in the western Pacific was on yellow alert, the middle phase of a three-step "traffic light" system comprising green, yellow and red levels.
"This means we should continue with our daily lives while preparing ourselves for any potential of an emergency," the government said in a statement, describing the risk to the island as "medium".
"It also means the government pulls its resources together through the emergency operations centre to ensure we are prepared to respond to emergencies."
Guam's emergency alert system, which sets off sirens across the island and interrupts regular television and radio broadcasts to deliver information, was tested at 10:00 am (0000 GMT) Thursday.
Governor Eddie Calvo said the test had been successful and the Guam Department of Homeland Security's joint information centre was now operating 24-hours a day to provide advice to the island's population of 180,000.
Homeland Security has described the chances of a North Korean attack as remote, noting that Guam "is protected by the greatest nation and military in the world, the United States of America".
But as a precaution, it launched a campaign this week urging residents to prepare emergency kits and family disaster plans.
Fact sheets have been distributed around the island advising residents on how to protect their families, stay informed and perform tasks such as blast-proofing doors and windows with plastic and tape.
Calvo said he had received assurances from US military commanders on the island, which bristles with American military facilities and bases, that defences were "strong and adequate".
"Even though we're protected, it's important we are vigilant," he said. "Look at the fact sheets, be prepared and stay tuned for news coming out of the joint information centre."
Guam, which came under US control in 1898 after the Spanish-American War, lies about 3,380 kilometres (2,100 miles) southeast of North Korea, meaning it is theoretically in range of Pyongyang's untested Musudan missiles.
Washington has scrambled to deploy its THAAD (Terminal High Altitude Area Defense) missile shield to Guam, a truck-mounted system capable of tracking and shooting down an enemy projectile.
Despite Pyongyang's menacing rhetoric, many islanders say their daily life has not been disrupted by the threat of a missile strike.
"At home, we discuss it from time to time but we are not making any preparations because we know that it's just sabre-rattling," Gina Tabonares-Reilly told AFP earlier this week.
"There are good enough missile interceptors underneath our waters... North Korea knows that and they are smart (enough) not to commit suicide."