Senate Democrats warned of Donald Trump’s mostly unchecked ability to launch nuclear strikes, expressing fears that the President could impulsively initiate a devastating attack.

“I would like to be able to tell my constituents and the American people we have a system in place that prevents an impulsive and irrational decision to use nuclear weapons,” Senator Ben Cardin, a Maryland Democrat, said at the outset of a Senate Foreign Relations Committee hearing. “Unfortunately, I cannot make that assurance today”.

Other Democrat colleagues of Mr Cardin said that Mr Trump is so "volatile" it ensured such a move could not be discounted.

“We are concerned that the President of the United States is so unstable, is so volatile, has a decision-making process that is so quixotic, that he might order a nuclear weapon strike that is wildly out of step with US national security interests,” Connecticut Democrat Chris Murphy said.

The hearing was convened by Tennessee Republican Bob Corker, who has broken with the President in a public and acrimonious spat that has divided the former allies. He warned in an October interview that Mr Trump risked steering the country toward “World War III”.

Despite that dire warning, Mr Corker deliberately did not couch a hearing of the committee he chairs on presidential nuclear powers as a rebuke to Mr Trump, saying “this is not specific to anybody”.

But Democrats on the committee were happy to invoke Mr Trump, and they noted that the President’s escalating rhetorical battle with North Korea — a nuclear-armed nation he and his advisers have repeatedly threatened to annihilate — lent urgency to their questions about how, if at all, presidents are limited in their abilities to fire nuclear missiles.

“This is not a hypothetical question,” Mr Cardin said, noting that a nuclear first strike on North Korea could be an alternative to a conventional military campaign that would produce mass casualties in Japan and South Korea.

Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Show all 6 1 /6 Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb Photos released by North Korea show Kim Jong-un talking to subordinates next to a device thought to be the new thermonuclear weapon. There is no way of independently verifying the pictures STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb North Korea claims it has successfully tested an advanced hydrogen bomb which could be loaded onto an intercontinental ballistic missile AFP/Getty Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A diagram on the wall behind Mr Kim shows a bomb mounted inside a cone STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb North Korean leader Kim Jong-Un (C) attending a photo session with participants of the fourth conference of active secretaries of primary organisations of the youth league of the Korean People's Army (KPA) in Pyongyang STR/AFP/Getty Images Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A new stamp issued in commemoration of the successful second test launch of the "Hwasong-14" intercontinental ballistic missile KCNA via Reuters Kim Jong-un inspects weapon North Korea says is powerful hydrogen bomb A new stamp issued in commemoration of the successful second test launch of the "Hwasong-14" intercontinental ballistic missile KCNA via Reuters

Military experts testifying before the committee noted that, while presidents have ultimate authority to order nuclear strikes, there are safeguards in place to ensure those orders are considered first.

Former Commander of US Strategic Command Gen C Robert Kehler noted that America’s system requires civilian oversight, meaning “This is a system controlled by human beings. Nothing happens automatically”. A nuclear first strike would need to meet certain legal requirements, he added, noting that the military is obligated to disobey an “illegal order”.

“The president would not make this decision by himself,” said Brian McKeon, a former acting undersecretary for policy with the Department of Defense. “The system for decision is designed to ensure the president consults with the national security council and his other senior civilian and military advisers and I would expect that to occur in every case where the use of nuclear weapons is contemplated”.

H R McMaster calls North Korea a threat 'to the entire world'

But they acknowledged that the President could overrule the advice of his advisers and order a nuclear strike if it is deemed lawful.

A prolific user of Twitter, Mr Trump has antagonized adversaries in 140-character bursts. A North Korean official dubbed a tweet by Mr Trump a "declaration of war" after the president said that North Korean officials “won’t be around much longer” if they continue with their escalating rhetoric over Pyongyang's nuclear program.

Multiple senators argued that the President’s loose Twitter finger could have catastrophic consequences. New Hampshire Democrat Jeanne Shaheen noted said she wanted “the President to act in a way that acknowledges input from a lot of experts and not to act based on a Twitter post”.

“I don’t think that the assurances that I’ve received today will be satisfying to the American people,” Massachusetts Democrat Ed Markey said. “I think they can still realize that Donald Trump can launch nuclear codes just as easily as he can use his Twitter account”.

Mr McKeon, the former Department of Defense official, echoed that view.