US President Donald Trump has accused his French counterpart Emmanuel Macron of being "nasty" for saying Nato was brain dead.
Mr Trump is in London for a summit marking the bloc's 70th anniversary.
At a news conference, Mr Trump said Nato served a great purpose and Mr Macron's remarks had been "very insulting".
He also said he could see France "breaking off" from Nato, but did not explain why.
Speaking last month, Mr Macron had complained that Nato members were no longer co-operating on key issues.
He described the alliance as "brain dead", stressing what he saw as a waning commitment from its main guarantor - the US.
The summit, due to get under way later on Tuesday, has already been overshadowed by a bitter row between member states France and Turkey, and a continuing debate over money.
Turkish President Recep Tayyip Erdogan has said he will oppose Nato's plan for the defence of the Baltic if it doesn't back Turkey over its fight against Kurdish groups it considers terrorists.
BBC
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