US President Donald Trump has launched a new attack on London Mayor Sadiq Khan, saying he is a "disaster" and will "only get worse" after three people were slain in the city in less than 20 hours.

A man in his 30s and an 18-year-old man were stabbed to death and a 19-year-old was shot dead in a series of violent incidents across the UK capital.

Police said 14 people, including several boys and a girl, had been arrested in connection with the incidents, which they did not believe were linked.

Mr Trump retweeted a tweet by British right-wing commentator Katie Hopkins, who called London "stab city" and said "this is Khan's Londonistan".

"London needs a new mayor ASAP. Khan is a disaster — will only get worse," Mr Trump said on Twitter.

A spokesman for Mr Khan said he was not going to waste time responding to Mr Trump's tweet.

Mr Khan said he was "sickened" by the deaths and London's Met Police said "bearing down on violent crime" in the capital remained "their top priority".

London's spike in knife attacks has been compared by anti-violence campaigners to a rapidly spreading virus.

Last year, Mr Khan said there were no quick fixes to the problem.

"To really make significant progress can take up to 10 years and a generation," Mr Khan told BBC radio.

Earlier this month Mr Trump called Mr Khan a "stone-cold loser" after the Mayor compared the US President to 20th-century fascists and criticised the British Government for inviting him for a state visit.

Mr Trump's feud with Mr Khan dates back several years.

Mr Khan has criticised Mr Trump's effort to ban travellers from Muslim countries, while the President has castigated the Mayor for his handling of a 2017 terrorist attack on the London bridge that killed 11 people.