Donald Trump has been criticised for claiming NATO troops stayed "a little off the front lines" in Afghanistan.
In an interview with Fox News in Davos, the US president repeated his criticism of NATO, saying he was not "sure" the alliance would "be there if we ever needed them".
He added: "We've never needed them.
"They'll say they sent some troops to Afghanistan … and they did, they stayed a little back, a little off the front lines."
Sky News US correspondent David Blevins said the comments would be deemed "grossly offensive" by allies who fought alongside the US.
America is the only NATO member to have invoked the collective security provisions of its Article 5 clause - that an attack against one member is an attack on all.
That came after the 9/11 terror attacks of 2001, which led to a US-led invasion of Afghanistan.
The UK suffered the second-highest number of military deaths in the Afghanistan conflict at 457. The US saw 2,461 deaths.
America's allies suffered 1,160 deaths during the conflict - around a third of the total coalition deaths.
'We have always been there'
Dame Emily Thornberry MP, chair of the Commons Foreign Affairs Select Committee, said Mr Trump's comments were "an insult" to the families of those who died and "so much more than a mistake".
Speaking on the BBC's Question Time, Dame Emily said: "How dare he say we weren't on the frontline, how dare he.
"We have always been there whenever the Americans have wanted us, we have always been there."
In a social media post, Liberal Democrat leader Sir Ed Davey MP said: "Trump avoided military service five times.
"How dare he question their sacrifice. Farage and all the others still fawning over Trump should be ashamed."
Trump's words 'a disservice' to troops
Tory MP Ben Obese-Jecty, who served in Afghanistan as a captain in the Royal Yorkshire Regiment, said it was "sad to see our nation's sacrifice, and that of our NATO partners, held so cheaply by the president of the United States".
He said: "I saw first hand the sacrifices made by British soldiers I served alongside in Sangin where we suffered horrific casualties, as did the US Marines the following year.
"I don't believe US military personnel share the view of President Trump; his words do them a disservice as our closest military allies."
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Last year, US vice president JD Vance hit back at claims of "disrespecting" British troops who served in Iraq and Afghanistan, after suggesting a potential peacekeeping force in Ukraine would be "20,000 troops from some random country that hasn't fought a war in 30 or 40 years".
At the time, only the UK and France had pledged troops to a potential peacekeeping force.
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