The US armed forces could refuse an order from the President to launch a nuclear strike if is deemed illegal, the country's top defence officer has said.
Air Force General John Hyten, commander of US Strategic Command, said he would reject any order from Donald Trump or any other president if it was not lawful.
Instead, he would offer to talk him through what options were legal, he told a security forum panel discussion.
The command would control nuclear forces in a war and would be the ones to launch any strike.
His comments come amid concern among Mr Trump's critics over whether the President has the judgement and temperament to be in charge of the nuclear codes.
Mr Trump has been in an escalating conflict with developing nuclear state North Korea in which both have threatened to unleash attacks.
His taunting tweets aimed at Pyongyang have sparked concerns, primarily among Democrats, that he may be inciting a war.
Speaking at the Halifax International Security Forum in Washington DC, General Hyten said he has discussed the laws over nuclear strikes with Mr Trump.
"I think some people think we're stupid. We're not stupid people," he said.
"We think about these things a lot. When you have this responsibility, how do you not think about it?,' he said.
Stressing he would not obey an illegal order, he added: "You could go to jail for the rest of your life."