Trump had suggested in an interview with Fox News this week that troops from NATO allies had avoided the front line in Afghanistan, even though 457 British soldiers were killed in the post-9/11 conflict. Starmer called those remarks "appalling."

"The GREAT and very BRAVE soldiers of the United Kingdom will always be with the United States of America!" Trump said on his Truth Social platform.

"In Afghanistan, 457 died, many were badly injured, and they were among the greatest of all warriors. It's a bond too strong to ever be broken."

Following the 9/11 attacks in 2001, the UK and several European countries joined the US in Afghanistan after it invoked NATO's collective security clause for the first and only time.

Soldiers from Canada, France, Germany, Italy, Denmark and others also died in the conflict.

"They'll say they sent some troops to Afghanistan," Trump said in the interview with Fox aired Thursday, referring to NATO allies.

"And they did, they stayed a little back, a little off the front lines," he added, triggering outrage across the political divide in Britain.

Trump also repeated his suggestion that NATO would not come to the aid of the United States if asked to do so.

On Friday, Starmer voiced his outrage.

"Let me start by paying tribute to 457 of our armed services who lost their lives in Afghanistan," Starmer said in a video message.

"There are many also who were injured, some with life-changing injuries, and so I consider President Trump's remarks to be insulting and frankly, appalling, and I'm not surprised they've caused such hurt to the loved ones of those who were killed or injured."

He said that if he had misspoken in such a way, he "would certainly apologize."

The White House initially rejected Starmer's comments and defended the president later Friday.

"President Trump is absolutely right - the United States of America has done more for NATO than any other country in the alliance has done combined," Taylor Rogers, a White House spokeswoman, said in a statement sent to AFP.