Kim Jong-un’s kingdom has shown an “unusual increase” in such activity recently, including air force flight operations. Jeong Kyeong-doo told of how South Korean officials had detected their neighbours’ change in behaviour amid the coronavirus pandemic. The defence chief said on Friday: “"North Korea ... has been heightening military tensions through an unusual increase in the inspection activities for its combat readiness posture, mostly of its artillery, and in its air force planes' flight operations.”

His warning comes as reports in Asia claimed Kim Jong-un may have died.

Mr Kyeong-doo said Pyongyang continues to refuse to hold peace talks with South Korea.

He said Kim Jong-un and his close circle have instead been concentrating on their arsenal of weapons.

North Korea has conducted five major weapons tests this year.

The hermit Asian nation has also carried out smaller-scale artillery firing drills.

Last week the military fired what appeared to be surface-to-ship cruise missiles off its east coast.

And on the same day Sukhoi-variant fighter jets and MiG-type planes were flown in the airspace above the city of Wonsan.

They fired several air-to-ground missiles into the East Sea.

On Sunday Foreign Secretary Dominic Raab, who is standing in for Boris Johnson, said he had no verified information on the rumours of Kim Jong-un’s supposed death.

The vice director of HKSTV Hong Kong Satellite Television [via the New York Post] cited a “very solid source” for the information.

But satellite images showing what is thought to be the dictator’s train at his holiday resort appear to dispel the rumour.

The snaps were reviewed by 38 North, a Washington-based North Korea monitoring group.

They said the images show the train had been parked outside the “leadership station” in Wonsan since at least April 21.

The monitors said the station is only used by members of the Kim family.

In a statement, 38 North said: “The train’s presence does not prove the whereabouts of the North Korean leader or indicate anything about his health but it does lend weight to reports that Kim is staying at an elite area on the country’s eastern coast.”