Donald Trump has been criticised for claiming NATO troops stayed "a little off the frontlines" 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 frontlines."
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.
Trump 'is plainly wrong'
Social care minister Stephen Kinnock said Mr Trump's claim was "deeply disappointing".
Mr Kinnock told Sky News: "President Trump's comments are deeply disappointing. There is no other way to say that, I don't know really why he said them. I don't think there's any basis for him to make those comments."
He said there is "a long history of the British armed forces standing shoulder to shoulder with the United States".
"We will always stand up for the values that we cherish: democracy, freedom, liberty to push back against dictators and anybody else that seeks to undermine our values and our way of life - and NATO is at the heart of that alliance."
He pointed to the fact that the only time NATO's Article 5 clause - a call for allies to come to a member's defence - had been activated "was to go to the aid of the United States after 9/11".
"And many, many British soldiers and many soldiers from other European and NATO allies gave their lives in support of American missions, American-led missions in places like Afghanistan and Iraq," he said.
"I am incredibly proud of our armed forces. They have put their lives on the line for our country. They are the definition of honour and valour and patriotism - and anybody who seeks to criticise what they have done and the sacrifices they make is plainly wrong," Mr Kinnock added.
'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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