Macron stood by his criticism of NATO and said its real problem was a failure to forge a clear purpose since the end of the Cold War.
London: US President Donald Trump and French leader Emmanuel Macron clashed over the future of NATO on Tuesday before a summit intended to celebrate the 70th anniversary of the Western military alliance.
In sharp exchanges underlining discord in a transatlantic bloc hailed by backers as the most successful military pact in history, Trump demanded that Europe pay more for its collective defence and make concessions to US interests on trade.
Macron, the French president, stood by comments he made last month describing NATO as suffering from a lack of strategic purpose akin to "brain death", and criticised fellow NATO member Turkey, which he accused of working with Islamic State proxies.
Washington and Paris have long argued over NATO's purpose - France opposed the 2003 Iraq war - but the new tensions will add to doubts over the alliance's future that have grown with Trump's ambivalence over US commitments to defend Europe.
Trump said Macron's criticism of NATO was "very, very nasty" and questioned whether the US military should defend any countries that were "delinquent" on alliance targets for national military spending.
"It's not right to be taken advantage of on NATO and also then to be taken advantage of on trade, and that's what happens. We can't let that happen," Trump said of transatlantic disputes on issues ranging from the aerospace sector to a European digital services tax on US technology giants.
All 29 member states have a target of spending 2 per cent of their gross domestic product on defence and Trump has singled out Germany for falling short of that goal.
But Macron stood by his criticism of NATO and said its real problem was a failure to forge a clear purpose since the end of the Cold War.
"If we invest money and put our soldiers' lives at risk in theatres of operation we must be clear about the fundamentals of NATO," he said in a tweet at the end of a day overshadowed by tensions between the French and US leaders.
A French presidency official said Trump often makes strident statements ahead of bilateral meetings and cools his rhetoric later. He noted that Macron and Trump "exchanged jokes and were very relaxed" at a joint news conference in London.
Collective defence at stake
Turkey threatened to block a plan to defend Baltic states and Poland against Russian attacks unless NATO backed Ankara in recognising the Kurdish YPG militia as terrorists.
The YPG's fighters have long been US and French allies against Islamic State in Syria. Turkey considers them an enemy because of links to Kurdish insurgents in southeastern Turkey.
"If our friends at NATO do not recognise as terrorist organisations those we consider terrorist organisations ... we will stand against any step that will be taken there," Turkish President Tayyip Erdogan said before travelling to London.
Erdogan has already strained alliance ties with a move to buy Russian air defence systems. Trump said he was looking at imposing sanctions on Ankara over the issue.
The uncertainty over the plan for Poland and the Baltic states, drawn up at their request after Russia annexed Crimea from Ukraine in 2014, raises issues about security on all of NATO's frontiers.
Under the North Atlantic Treaty Organisation's 1949 founding treaty, an attack on one ally is an attack on all its members, and the alliance has military strategies for collective defence across its territory.
The summit, in a hotel in Hertfordshire just outside London, begins on Wednesday.
On Tuesday evening, alliance leaders attended a reception hosted by Queen Elizabeth at Buckingham Palace.
The British monarch, in a teal-coloured matching jacket and skirt, greeted the summiteers and accompanying partners, including former fashion model Melania Trump, who was wearing a bright yellow dress with matching cape and purple sleeves.
They were then welcomed to 10 Downing Street by British Prime Minister Boris Johnson, host of the summit a little over a week before the country faces an election.
Several hundred protesters gathered in London's Trafalgar Square, holding placards reading: "Dump Trump" and "No to racism, no to Trump". A police line divided them from a small group of Trump supporters wearing Make America Great Again caps, waving American flags and shouting: "Build the wall".
In Washington on Tuesday, Democrats in the US House of Representatives laid out their impeachment case against Trump, accusing him of using the powers of his office to solicit foreign interference in the 2020 election.
Hoping to placate Trump, Europe, Turkey and Canada will pledge at the summit some USD 400 billion in defence spending by 2024, and agree to a reduction of the US contribution to fund the alliance itself.
The allies will approve a new strategy to monitor China's growing military activity, and identify space as a domain of warfare, alongside air, land, sea and computer networks.
Trump said he believed Russia wanted deals on arms control and nuclear issues, and that he would be willing to bring China into such accords.