US President Donald Trump caused anger in France and Britain by suggesting looser gun laws could have helped prevent deadly attacks in Paris in 2015 and linking 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 and one minister urged the leader to apologise, 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 November 13, 2015 and demands that the memory of the victims be respected," the foreign office said.

French Finance Minister Bruno Le Maire said he hoped Trump "would come back on his words and express regret".

"His comments are shocking and not worthy of the president of the world's greatest superpower," Le Maire told BFM television yesterday.

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

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 knife attack victims.

"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 said, making stabbing gestures.

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