A US military assault on North Korea could prompt a last gasp retaliatory nuclear attack by Pyongyang on Seoul and Japan killing as many as 3.8 million people, according to a new analysis of the destructive potential of Kim Jong-un arsenal.

Donald Trump has repeatedly warned that all options remain on the table – including the use of force – in dealing with the North Korean threat.

However, analysts say the US is hamstrung by Pyongyang’s growing weapons stockpile and the fact that major population centres lie well within its range.

A detailed analysis by 38 North, a programme at the US-Korea Institute at Johns Hopkins School of Advanced International Studies, has crunched the numbers, based on North Korea’s likely arsenal of as many as 25 weapons with possible yields of up to 250 kilotons, of the sort tested last month.

Using population density figures and blast radius estimates, Michael Zaguruk, calculated that a single detonation over Seoul could kill almost 800,000 people and over Tokyo would kill almost 700,000.