HAGATNA, Guam (AFP) - 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 is the same colour used by government agencies to indicate there is a medium risk for the island," the government said in a statement released late on Wednesday.

"The government will continue to operate like normal, with a few government agencies participating in emergency preparedness and planning."

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 (8.00am on Thursday, Singapore time).

The government also distributed fact sheets advising people how to protect their families and make doors and windows blast-proof with plastic and tape in the event of an emergency.

Governor Eddie Calvo said he had received assurances from US military commanders on the island of 180,000 people, which is dotted with American military facilities, 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 lies about 3,380 kilometres southeast of North Korea, meaning it is theoretically in range of Pyongyang's Musudan missiles, although it remains unclear whether the weapons actually work.