US President Donald Trump and French leader Emmanuel Macron have clashed about the future of NATO 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.
President Macron 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 the target.
But Macron, speaking later at a news conference with Trump, stood by his criticisms of NATO and said its real problem was a failure to forge a clear purpose since the end of the Cold War.
"We have to be clear on what the fundamentals of NATO will be, which is not the case now," he said.
Adding that NATO has not even agreed a definition of terrorism, he said, "When I look at Turkey, they are fighting against those who fought with us shoulder to shoulder against ISIS (Islamic State) and sometimes they work with ISIS proxies."
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, but Turkey considers them an enemy because of links to Kurdish insurgents in its southeastern region.
"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.
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.
In a bid to placate Trump, Europe, Turkey and Canada will pledge $US400 billion ($A584 billion) in defence spending by 2024, and also agree to reduce the US contribution to fund the alliance itself.
The allies will approve a new strategy to monitor China's growing military activity, and name space as a domain of warfare, alongside air, land, sea and computer networks. They will issue a statement condemning Moscow's Crimea annexation.
On Tuesday evening, leaders were due to attend two receptions, one hosted by Queen Elizabeth and one by Prime Minister Boris Johnson.
The summit, in a hotel in Hertfordshire just outside London, begins on Wednesday.