South Korea has pledged to develop a powerful “Frankenmissile” capable of destroying North Korea’s underground military facilities and wartime commands as it beefs up its armoury to counter the growing threat from across the border.

According to the Korea Herald, military sources in Seoul revealed the plan the day after South Korean President Moon Jae-in and his US counterpart Donald Trump agreed to scrap an imposed 500kg limit on the warheads fitting onto the South’s missiles.

Mr Moon had told the US president during a telephone call on Monday, after Pyongyang’s detonation of a hydrogen bomb test, that it was “crucial” to find “powerful and practical measures” to give North Korea a wake-up call about the consequences of its actions.