Guam residents received a nasty jolt yesterday after two radio stations accidentally issued emergency warnings to indicate an imminent threat or attack, at a time when the US territory is already on edge over North Korean threats to fire missiles into nearby waters.

Several concerned listeners were reported to have called police after the stations triggered the Emergency Alert Broadcast System, issuing "a civil danger" warning at 12:25 am that was later confirmed to be a mistake. "Residents and visitors are reminded to remain calm," said Guam's homeland security adviser George Charfauros. "There is no change in threat level, we continue business as usual."

Homeland Security confirmed in a statement that the "unauthorized test was not connected to any emergency, threat or warning" and it was working with the radio stations "to ensure the human error will not occur again".

North Korea offered a glimpse into its plans to fire missiles near Guam in photos released by its state media on Tuesday, as leader Kim Jong Un was briefed on the plans drawn up by the army amid heightened tensions with the United States.

Kim was seen holding a baton and pointing at a map reading "Strategic Force's Firing Strike Plan", which showed a flight path for the missiles appearing to start from North Korea's east coast, then flying over Japan and ending near Guam, as Pyongyang announced last week.

The launch location seen in the map appeared to be in the vicinity of Sinpo, the east coastal city that hosts North Korea's submarine base, said Kim Dong-yub, a military expert at Kyungnam University's Institute of Far Eastern Studies in Seoul.

Kim said the location near Sinpo fits with what North Korea outlined last week - that four intermediate-range missiles will cross the sky above Shimane, Hiroshima and Koichi Prefectures of Japan, fly 3,356.7 km (2,085.8 miles) for 1,065 seconds and hit the waters 30 to 40 km (18 to 25 miles) from Guam.

"Every North Korean must have seen this photo on TV and newspapers. North Korea is showing its confidence, telling the United States: if they want to stop it they can try," he said. "It also signals that the North has been studying this for a long time and getting ready to act if it decided to."

Tension on the Korean peninsula has risen over North Korea's nuclear weapons program, with North Korea and the United States exchanging a flurry of strong rhetoric, each threatening military action, in recent days.

Defense Secretary Jim Mattis warned on Monday that the U.S. military would be prepared to intercept a missile fired by North Korea if it was headed to Guam, while the North's leader Kim Jong Un alerted his army that it should always be fire-ready.

In 2013 when tensions ran high as North Korea threatened missile strikes on U.S. Pacific bases, including Guam and Hawaii, it also released photographs of Kim inside his military command centre signing the order to put rockets on standby to attack the U.S. mainland. The pictures then showed a large chart titled "U.S. mainland strike plan" and it was seen targeting U.S. regions such as Hawaii, Washington D.C., and Texas. North Korea has never carried out its threats to hit U.S. Pacific bases or mainland.