But North Korea already has between six and eight nuclear warheads that could be mounted on a missile. If the regime does perfect a submarine-launched system then it would, in theory, be able to launch a nuclear attack on the American mainland. This would require a submarine being able to sail within missile range of the US.

Mr Obama gave warning of the possible consequences. “We could, obviously, destroy North Korea with our arsenals,” he told CBS News. “But aside from the humanitarian costs of that, they are right next door to our vital ally, [South] Korea."

Mr Obama said that America was improving its own missile defences. "One of the things that we have been doing is spending a lot more time positioning our missile defence systems, so that even as we try to resolve the underlying problem of nuclear development inside of North Korea, we're also setting up a shield that can at least block the relatively low level threats that they're posing right now," he said