North Korea has reportedly conducted mass evacuation drills as it prepares for the possibility of war.

South Korea-based NK News said several sources had reported the drills were being conducted in "secondary and tertiary cities and towns" over the last week.

None were observed in the capital Pyongyang and most were carried out on the pariah state's east coast, which borders the Sea of Japan.

Blackout drills, where towns turn off all light sources at night to avoid illuminating targets for the enemy, were also conducted.

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“I have never heard of this type of training exercises before in North Korea, but am not surprised,” former South Korean three-star lieutenant general LTG Chun In-bum told NK News. "They must realise how serious the situation is.

"In this day and age of precision bombing and all-weather capabilities, [such] North Korean civil exercises either come from ignorance of modern war or just a propaganda campaign to frighten their own people and induce obedience.”

One anonymous source said they had "never heard of evacuation exercises happening before."

Another said: “There used to be air raid drills in 2003, but not since then.

“A mass evacuation would be impossible not to notice.”

Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Show all 6 1 /6 Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Photos released by North Korea show Kim Jong-un talking to subordinates next to a device thought to be the new thermonuclear weapon. There is no way of independently verifying the pictures STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb North Korea claims it has successfully tested an advanced hydrogen bomb which could be loaded onto an intercontinental ballistic missile AFP/Getty Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A diagram on the wall behind Mr Kim shows a bomb mounted inside a cone STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb North Korean leader Kim Jong-Un (C) attending a photo session with participants of the fourth conference of active secretaries of primary organisations of the youth league of the Korean People's Army (KPA) in Pyongyang STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A new stamp issued in commemoration of the successful second test launch of the "Hwasong-14" intercontinental ballistic missile KCNA via Reuters Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A new stamp issued in commemoration of the successful second test launch of the "Hwasong-14" intercontinental ballistic missile KCNA via Reuters

The threat North Korea launching a nuclear missile attack is accelerating, US Defence Secretary James Mattis has said.

General Mattis accused the North of illegal and unnecessary missile and nuclear programmes and pledged to repel any strike.

"North Korea has accelerated the threat that it poses to its neighbors and the world through its illegal and unnecessary missile and nuclear weapons programs," he said, adding that US-South Korean military and diplomatic collaboration had thus taken on "a new urgency."