The shocking total was calculated using computer models to determine the impact of Kim Jong-un’s nuclear arsenal on the two densely populated capitals.
Tensions on the Korean Peninsula are running high as the hermit state shows no sign of slowing in its pursuit of more powerful nukes, as well as the long-range missiles needed to deliver them to their targets.
Because of the top secret nature of North Korea’s nuclear programme, it is impossible to predict exactly how much damage dictator Kim could cause if he pressed the button.
But Michael Zagurek Jr, an expert on the effects of nuclear weapons, has run a series of simulations based on everything from conservative estimates to ‘worst case scenario’ numbers.
He based his models on a situation where the North, under attack from the United States, launched its entire arsenal against the South Korean and Japanese capitals.
In a report for North Korean watchdog 38 North, Mr Zagurek writes: “The US carrying out any military option raises a significant risk of military escalation by the North, including the use of nuclear weapons against South Korea and Japan.
“…If the ‘unthinkable’ happened, nuclear detonations over Seoul and Tokyo with North Korea’s current estimated weapon yields could result in as many as 2.1 million fatalities and 7.7 million injuries.”
Experts believe the North has the capability to launch nuclear-tipped ballistic missiles, and has already produced between 20 and 25 warheads, Mr Zagurek said.
These weapons are estimated to have the destructive power of around 20 kilotons.
But despot Kim’s test of a hydrogen bomb in September had an estimated yield of at least 108 kilotons – with some estimates putting it as high as 250.
By comparison, the nuclear bomb that devastated Hiroshima in the final days of World War Two had a yield of around 15 kilotons.
Mr Zagurek said: “With possible thermonuclear yields [and between 20 and 25 missiles], the number of deaths could range between 1.3million and 3.8million.”
The United States has deployed its THAAD missile defence system in South Korea, a move met by anger from North Korea.
The high-tech kit is designed to shoot ballistic missiles out of the sky without detonating any warheads attached to them.
Mr Zagurek said he had factored some of the North Korean missiles being intercepted in to his various casualty projections.