THE top US defence officer has said an order from President Donald Trump or any of his successors to launch nuclear weapons can be refused if that order is determined to be illegal.

Air Force General John Hyten, commander of US Strategic Command, told the Halifax International Security Forum that he would tell Trump he could not do an illegal strike.

“If it’s illegal, guess what’s going to happen. I’m going to say, ‘President, that’s illegal’. And guess what he’s going to do? He’s going to say, ‘What would be legal?’” Gen Hyten said.

“And we’ll come up with options with a mix of capabilities to respond to whatever the situation is, and that’s the way it works.”

In the event that Trump decided to launch a nuclear attack, Gen Hyten would provide him with strike options that are legal.

The command would control nuclear forces in a war. The comments come as nuclear attack threats from North Korea remain a serious concern and Trump’s critics doubt his temperament.

Trump’s taunting tweets aimed at Pyongyang have sparked concerns primarily among congressional Democrats that he may be inciting a war with North Korea.

During testimony before the Foreign Relations Committee earlier this month, retired General Robert Kehler, who served as the head of Strategic Command until 2013, also said the US forces have to follow legal orders