US President Donald Trump sparked anger in France and Britain by suggesting looser gun laws could have helped prevent deadly attacks in Paris in 2015, and linking a wave of knife crime in London to a handgun ban.

In a speech to the National Rifle Association (NRA) on Friday, Trump mimicked the shooting of victims in the Paris rampage, and said if civilians had been armed "it would have been a whole different story."

The French government issued its strongest criticism of Trump since he took office, at a time when President Emmanuel Macron has been reinforcing bilateral ties following a state visit.

"France expresses its firm disapproval of President Trump's comments about the Paris attacks on Nov. 13, 2015 and demands that the memory of the victims be respected," the foreign office said in a statement.

"Every country freely decides on its own laws on carrying firearms, as in other areas. France is proud to be a country where acquiring and carrying firearms is strictly regulated."

Other French politicians including the mayor of Paris took issue with Trump's comments, after he acted out the scene of the massacre by Islamist assailants at Paris' Bataclan concert hall, where 90 of the 130 victims of the attacks died.

"They took their time and gunned them down one by one. Boom! Come over here. Boom! Come over here. Boom!," Trump said, using his hands in a gun gesture.

Former French president Francois Hollande, who was head of state at the time, said on Twitter that Trump's comments and antics were "shameful" and "obscene".

'KNIVES, KNIVES, KNIVES, KNIVES'

Trauma surgeons in London, meanwhile, said Trump had missed the point after, in the same speech, he linked knife crime there to an absence of guns.

Comments by Trump have caused upset before in Britain. Relations with Prime Minister Theresa May cooled last year after she criticised him for retweeting anti-Islam videos by a British far-right group.

Trump, who is due to visit Britain on July 13, told NRA members that a "once very prestigious" London hospital, which he did not name, had become overwhelmed with victims of knife attacks.

"They don't have guns. They have knives and instead there's blood all over the floors of this hospital," he said. "They say it's as bad as a military war zone hospital. Knives, knives, knives, knives," he added, making stabbing gestures.

London suffered a spike in knife crime early this year, and saw more murders during February and March than New York.

Last month, trauma surgeon Martin Griffiths told the BBC that some of his colleagues had likened the Royal London Hospital in east London where he works to the former British military base Camp Bastion in Afghanistan.

But on Saturday he indicated Trump had drawn the wrong conclusion from his remarks.

Griffiths posted his comment next to an animation of a stick figure with the phrase "The Point" flying over its head, and also linked to a statement on the hospital's website by a fellow trauma surgeon, Karim Brohi.

"There is more we can all do to combat this violence, but to suggest guns are part of the solution is ridiculous. Gunshot wounds are at least twice as lethal as knife injuries and more difficult to repair," Brohi said in the statement on Saturday.

Britain's government effectively banned handgun ownership in England, Scotland and Wales after a school shooting in 1996.