US president questioned by congress on nuclear authority

The Senate Foreign Relations Committee hearing is titled Authority to Order the Use of Nuclear Weapons.

The chairman of the panel accused President Trump last month of setting the US "on a path to World War III".

In August, Mr Trump vowed to unleash "fire and fury like the world has never seen" on North Korea 

if it continued to expand its atomic weapons programme. Senators also wanted to know what would happen if the president ordered a nuclear strike.

Robert Kehler, an ex-commander of US Strategic Command, said that in his former role he would have followed the president's order to carry out the strike - if it were legal.

Mr Kehler said if he were uncertain about its legality, he would consult with his own advisers.

Under certain circumstances, he explained: "I would have said, 'I'm not ready to proceed.'"

As commander-in-chief, the president has the sole authority to order a nuclear attack, which can be delivered either by submarine, airplane, or intercontinental ballistic missile (ICBM)- which make up the so-called "nuclear triad".

Under current rules, the US president could launch a strike by entering the codes into a device called "the football", which travels everywhere with the president.

Mr Trump is not required to consult anyone or gain consensus from any other members of government.

His top advisers, such as Defence Secretary James Mattis, Secretary of State Rex Tillerson, or national security adviser Lt Gen HR McMaster, play no role in the chain of command.

Congressional approval is required for the use of conventional military force.