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. Kim Jong-un with regime engineers at an undisclosed location Credit: Korea News Service via AP Those numbers rise to a combined total of almost four million dead if North Korea manages to launch multiple rockets carrying its latest thermonuclear bomb. And that is without considering the effects of radioactive fallout. The numbers are a sobering reminder of the difficulties of dealing with Mr Kim and his nuclear arsenal. Mr Zaguruk explains the possible measures available to the US and the possible response. “This could include such options as attempting to shoot down the test missiles or possibly attacking North Korea’s missile testing, nuclear related sites, missile deployment areas or the Kim regime itself,” he writes. “The North Korean leadership might perceive such an attack as an effort to remove the Kim family from power and, as a result, could retaliate with nuclear weapons as a last gasp reaction before annihilation.” US discussions of military options raises risk of military escalation by #NorthKorea–Michael Zagurek shows human cost if DPRK were to launch #nuclear attack on Seoul or Tokyohttps://t.co/jescZwcTNX— 38 North (@38NorthNK) October 4, 2017 Boris Johnson, the Foreign Secretary, admitted the scale of the problem last month. He said: “It’s certainly our view that none of the military options are good…the distance between North Korea and Seoul is very, very small and they could basically vaporise large parts of the South Korean population even with conventional weapons, so that’s not really very easy to threaten or to deliver.” Boris Johnson condemns North Korea's latest missile test 01:30 Meanwhile, North Korea continues to press ahead with its nuclear and missile programmes. Fresh reports circulated on Friday that a long-range missile, capable of reaching the west coast of the US, was being prepared for testing. Anton Morozov, a member of the Russian lower house of parliament's international affairs committee, made the claim after returning from a visit to North Korea. "They are preparing for new tests of a long-range missile. They even gave us mathematical calculations that they believe prove that their missile can hit the west coast of the United States," he said, according to the RIA news agency. "As far as we understand, they intend to launch one more long-range missile in the near future. And in general, their mood is rather belligerent."