US President Donald Trump has accused his French counterpart Emmanuel Macron of being "nasty" for saying Nato was "brain dead", reports BBC News.
Mr Trump is in London for a summit marking the Western military alliance's 70th anniversary.
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.
Nato - the North Atlantic Treaty Organisation - was founded after World War Two to counter the threat of Soviet expansion. The 29 member states pledge to come to the aid of one another should any come under attack.
But speaking last month, Mr Macron 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 biggest 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 region if it does not back Turkey over its fight against Kurdish groups it considers terrorists.
Sitting alongside Nato Secretary General Jens Stoltenberg at a news conference, President Trump said Nato "serves a great purpose", but was then asked what he thought about Mr Macron's statements.
He said he thought the French leader had been "very disrespectful" to other alliance members.
"It is a very, very nasty statement. I think they have a very high unemployment rate in France. France is not doing well economically at all," he said.
"It is a very tough statement to make when you have such difficulty in France when you look at what is going on. They have had a very rough year. You just can't go around making statements like that about Nato. It is very disrespectful."