Guam's governor Eddie Calvo says it is prepared for the worst despite official warnings there is only a one in a million chance of a nuclear missile ever reaching Guam.
North Korea has said the military has a plan ready to launch four rockets which could land between 30 and 40 kilometres of the island.
Radio stations in the US territory reportedly tested an emergency alert early this morning but the authorities say the threat level remains the same.
Homeland security has been distributing flyers with advice on what to do in the event of a nuclear attack including precautions like not looking at a flash or fireball.
Guam is strategically important to the United States, housing both an Air Force and Navy base. Here, a US F-16 flies along the island's coastline. Photo: US Department of Defense
Mr Calvo told media the measures were purely precautionary.
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"Though the threat level has never changed, you have to provide this information," Mr Calvo said.
"It's not different messages, it is understanding where we are at right now.
"But then again, even if there is a one in a million chance, it is the job of the leader of the government to ensure that we are properly prepared for any eventuality," he said.