The commander of America’s nuclear forces says he would resist any “illegal” presidential order to launch a strike.

General John Hyten made his comments after Democrats raised concerns about President Donald Trump’s volatile behaviour and his escalating war of words with North Korea.

Gen Hyten said he had spent years studying US laws of conflict and the criteria that a sitting president must consider before ordering a nuclear strike.

“If it’s illegal... I’m gonna say, ‘Mr President, that’s illegal.’ And guess what he’s gonna do? He’s gonna say, ’What would be legal?’ And we’ll come up with options, of a mix of capabilities, to respond to whatever the situation is,” he told the Halifax Security Forum in Canada after being asked about the conversation he would have with Mr Trump on a potential strike.

Gen Hyten heads US Strategic Command, which is responsible for American cyber capabilities, missile defence and nuclear capabilities.