Boris Johnson has said the NHS was not discussed in his meeting with President Trump on Tuesday.
The US president responds to a viral video of Canadian leader Justin Trudeau appearing to mock him.
Leaders of the Nato member states are in London for the alliance's 70th birthday, and things are tense.
From a Royal moment to gossiping world leaders, not everything at Nato's UK gathering has followed script.
UK Prime Minister tells Nato leaders the alliance must be ready to confront emerging threats.
UK Prime Minister Boris Johnson has said Nato continues to provide "peace and prosperity for hundreds of millions of people."
The prime minister arrived at the Nato leaders meeting in Watford earlier this morning, which marks the 70th anniversary of the alliance.
Speaking on the way in, Prime Minister Boris Johnson said Nato continued to provide "peace and prosperity for hundreds of millions of people" because of "a very simple concept of safety in numbers."
"At the heart of it is a pledge that we will come to one another's defence - all for one and one for all."
But Mr Johnson warned Nato "cannot be complacent". He said the UK would continue to invest 2% of its GDP in defence and its commitment to the alliance was "absolutely rock solid".
The BBC's Jessica Parker explains why Donald Trump is talking about the NHS, and Tuesday's election campaigning.
French President Emmanuel Macron said France pays with its "soldier's lives".
The Lib Dem leader says she'd work with Trump in government as you have to work with those you disagree with.