US President Donald Trump has taken aim at London Mayor Sadiq Khan, calling him a 'national disgrace'.

Three people were killed in a small area of London just hours apart in separate attacks. 

Police have arrested 14 people after a torrid 24 hours of violence in the city.

One teenager was killed in Wandsworth, in south west London, while a second was shot dead in Plumstead, on the other side of the city. A third man was stabbed to death in Poplar in east London.

London mayor Sadiq Khan says he's sickened by the deaths and has condemned what he calls the "scourge of violence" in the city.

LONDON needs a new mayor ASAP. Khan is a disaster - will only get worse! https://t.co/n7qKI3BbD2

— Donald J. Trump (@realDonaldTrump) June 15, 2019

However, the US President has wavered into the debate, writing on Twitter: "LONDON needs a new mayor ASAP. Khan is a disaster - will only get worse!" as he retweeted a post from right-wing commentator Katie Hopkins who referred to the capital as "stab city".

In a follow-up attack on Khan, Trump wrote: "He is a national disgrace who is destroying the City of London."

Many Twitter users wandered into the debate, including Kiwi comedian Jemaine Clement, who pointed out that four of the world's top 50 cities in terms of homicide rates were in the US, while none are in the UK. 

Khan declined via a spokesperson to "waste his time" responding to the comments. 

Earlier this month, Trump called Khan a "stone-cold loser" after the Mayor was critical of the President ahead of his UK state visit.