US President Donald Trump and French leader Emmanuel Macron have set out opposing views ahead of a Nato summit.
In an occasionally tense press conference, the two politicians sparred over Nato's role, Turkey, and Islamic State group (IS) fighters.
Mr Trump had described Mr Macron's comments about Nato as "nasty", but Mr Macron said he stood by his words.
World leaders are in London to mark the Western military alliance's 70th anniversary.
The summit has already been marked by strained relations between Turkey and other member states.
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.
On Tuesday night, Mr Macron and Mr Erdogan met in Downing Street in a four-way meeting that also included German Chancellor Angela Merkel and the host, UK Prime Minister Boris Johnson.
Why is there a row over Nato?
Ties between Mr Trump and Mr Macron were already strained amid a trade dispute, and after the French president described Nato as "brain dead" last month because, he said, the US commitment to the alliance was fading.
Mr Trump hit back on Tuesday by saying the French leader had been "very disrespectful", adding that France had "a very high unemployment rate" and "nobody needs Nato more than France".
At a joint press conference with Mr Macron later, Mr Trump was less combative, stressing that the two countries had "done a lot of good things together". Mr Macron, meanwhile, said he stood by his comments.
The two sides then clashed over foreign IS fighters who were captured in Syria.
Mr Trump jokingly offered them to France, saying: "Would you like some nice [IS] fighters? You can take everyone you want."
Sounding stern, Mr Macron said "Let's be serious" and that IS fighters from Europe were "a tiny minority", and that the "number one priority" was to get rid of the terrorist group.
Mr Trump then retorted: "This is why he is a great politician because that was one of the greater non-answers I have ever heard, and that's OK".
Mr Trump also criticised Nato countries who were paying less than the Nato guidelines of at least 2% of GDP towards the alliance.
He said he did not want countries to be "delinquent" and pay less than their share, adding: "Maybe I'll deal with them from a trade standpoint."
Mr Macron said France - which currently spends 1.84% of its GDP on defence - would reach the minimum, and acknowledged that the US had "overinvested" in Nato for several decades.
However, he added that there were other pressing issues to discuss.
"When I look at Turkey, they now are fighting against those who fought with us shoulder to shoulder against [IS]... if we just have discussions about what we pay and we don't have clear discussions about such a situation, we are not serious."
The two leaders also discussed Turkey's decision to buy a Russian S-400 missile system.
Mr Trump said they were "looking at" whether to impose sanctions, while Mr Macron asked: "How is it possible to be a member of the alliance... and buy things from Russia?"
Why is Turkey an issue?
The Kurdish-led Syrian Democratic Forces (SDF) have been key allies of the US-led coalition against IS in Syria. However, Turkey views a section of the group - the YPG - as terrorists.
Ahead of his departure for London, Mr Erdogan said Turkey would not approve a plan to defend Poland, Lithuania, Latvia and Estonia in the event of a Russian attack unless Nato recognised the Kurdish YPG militia as terrorists.
"If our friends at Nato don't recognise as terrorist organisations those we consider terrorist organisations... we will stand against any step that will be taken there," he said about the plan.
However, Mr Macron told reporters: "We don't have the same definition of terrorism around the [Nato] table".
In October, Turkey launched an operation in Kurdish-controlled areas of northern Syria to create a "security zone" along its border.
That military action deepened fractures between Turkey and other Nato members, and took place after President Trump had controversially pulled US forces out of the region.
What is Nato?
- The North Atlantic Treaty Organisation is the world's most powerful defence alliance
- It was set up in 1949, after World War Two, amid anxiety over Soviet expansion
- It was founded on the principle of collective defence between allies
- Originally had 12 members, but now has 29