Donald Trump's administration has told the UK to "reject two-tier policing" as the US sent its condolences to Henry Nowak's family.
The State Department, which is headed up by Marco Rubio, released a 41-word statement on Nowak's murder on social media.
In the statement, the State Department said: "Ideological conditioning and two-tiered policing are glaring symptoms of civilisational decline.
"They must be rejected across the West. The United States sends our condolences to the family of Henry Nowak and the people of the United Kingdom at this troubling time."
Mr Trump's relationship with Sir Keir Starmer has become increasingly fraught over the past six months, with the pair disagreeing over Greenland, Iran and the Chagos Islands.
The US President has also lashed out at the Prime Minister over free speech, North Sea oil and immigration.
The State Department's intervention also echoes Reform UK leader Nigel Farage's concerns following Nowak's murder.
Nowak, an 18-year-old finance student from Essex, was stabbed five times on the way home from a night out in December last year.
Marco Rubio's State Department released a statement on Henry Nowak's murder this evening
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When officers arrived at the scene, Nowak told them he had been stabbed and that he could not breathe.
An officer was heard dismissing Nowak's cries for help, telling the teenager: "I don't think you have, mate."
Hampshire & the Isle of Wight Constabulary later revealed that officers had been misled by Sikh killer Vickrum Digwa.
Digwa, who was sentenced to life with a minimum tariff of 21 years, lied by claiming he had been the victim of a racist attack.
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Henry Nowak should be starting his second year at university | PAMr Farage, who was accused of inciting unrest by calling for "cold rage", claimed Nowak's murder was an example of "two-tier policing".
However, Sir Keir rejected the suggestion at Prime Minister's Questions this week.
He said: “I don’t believe there’s two-tier policing in this country."
Taking aim at the Reform UK leader, the Prime Minister added: “His response has been to appeal for rage. Rage. That’s his response to a father who’s lost his son and asked for that not to happen.
Keir Starmer met Henry Nowak's family privately in Downing Street amid their plea not to 'whip up division'
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"Exploiting this tragedy to create grievance and division would be wrong in any circumstances but to do when the family are expressly saying ‘please don’t’ is unforgivable. It shows exactly who he is.”
Speaking to GB News today, Mr Farage condemned the violence seen in Southampton while doubling down on "two-tier" policing.
He said: "Of course, I condemn all violence, I always have, but what they don't want to listen to in the Commons is that if you don't get rid of the perception, reality, or both of two-tier policing, you finish up with lots and lots of angry young, white working-class men."
Defending his remarks earlier this week, the Reform UK leader added: "I defy any fair-minded people to watch that video and not be angry.
Henry Nowak was stabbed nine times by Vickrum Digwa who then claimed to officers that Henry had racially abused and assaulted him | FACEBOOK"So, I wanted to express it as cold rage. Not hot anger, not violent outbursts.
"Cold rage means that you're boiling inside about it, but you're not in any way, and you've known me long enough.
"I've never, in 35 years of being in politics, advocated for people going outside the law. Not once."
However, Cabinet Office Minister Nick Thomas-Symonds claimed Mr Farage's response proved he is not fit to be Prime Minister.