* NATO leaders meet at luxury golf resort for formal summit
* Defence spending, China, NATO reform on the agenda
* Turkey, France split over strategy in Syria
* President Trump remains tough on call for more spending (Adds details on Buckingham Palace reception, quotes)
By Robin Emmott and William James
WATFORD, England, Dec 4 (Reuters) - NATO leaders gathered at a golf resort near London on Wednesday for a summit acrimonious even by the standards of the Trump era, aiming to tackle sharp disagreements over spending, future threats including China and Turkey's role in the alliance.
With U.S. President Donald Trump and French President Emmanuel Macron blowing hot and cold over NATO's function, the 29-member military alliance is looking for reinvigoration as it marks the 70th anniversary of its Cold War-era founding.
"Clearly it is very important that the alliance stays together," British Prime Minister Boris Johnson told reporters as he prepared to welcome heads of state and government. "But there is far, far more that unites us than divides us."
Leaders held preliminary meetings in London on Tuesday, during which stark differences were aired, with Trump, who in the past has called NATO obsolete, criticising Macron for comments last month about NATO's strategic "brain death".
Trump said Macron's remarks were "nasty". He also described allies who spend too little on defence as "delinquent".
Macron held his ground, saying as he arrived that it was important for leaders to discuss issues in an open and forthright manner if they were to find solutions.
"I think it's our responsibility to raise differences that could be damaging and have a real strategic debate," he said. "It has started, so I am satisfied."
Earlier, in a message on Twitter, Macron was direct about the challenges NATO faces. "It is a burden we share: we can't put money and pay the cost of our soldier's lives without being clear on the fundamentals of what NATO should be," he said.
In an illustration of the awkward mood, Macron, Johnson and the prime ministers of Canada and the Netherlands were caught on video at a Tuesday evening Buckingham Palace reception, apparently making light of Trump's media appearances.
"It was like a 40-minute press conference," Canada's Justin Trudeau can be heard saying, with Queen Elizabeth's daughter Princess Anne listening on. "I just watched his team's jaws drop to the floor," Trudeau added with a chuckle.
One of Macron's chief complaints is that Turkey, a NATO member since 1952 and a critical ally in the Middle East, has increasingly acted unilaterally, carrying out incursions into Syria, taking up arms against the Kurdish YPG militia that had been allied with Western forces against Islamic State, and buying the S-400 missile defence system from Russia.
Turkish President Tayyip Erdogan has pushed back, saying he will oppose NATO's plan for the defence of Baltic countries if the alliance does not recognise groups that Turkey deems terrorists, including the YPG.
As he arrived at the summit, his back stooped, Erdogan declined to speak. His increasingly close ties with Russia, particularly over Syria, and his differences with the European Union over migration among other issues, have made him a more difficult NATO partner and, conversely, a more essential one.
Arriving at the 18th-century estate that once hosted a golf championship won by Tiger Woods, Estonian Prime Minister Juri Ratas - whose country depends on NATO as a shield against Russia - said he was confident divisions could be overcome.
"NATO is strong. NATO's deterrence is 100% credible," he said. "Transatlantic ... cooperation is a cornerstone for us, for our security, for both sides of the Atlantic."
At the summit, Europe, Turkey and Canada are expected to respond to Trump's accusations that they spend too little on defence by pledging an extra $400 billion by 2024. Germany, a frequent target of Trump's blandishments to spend more, has promised to spend 2% of national output by 2031.
France and Germany want the alliance to consider a bigger role in the Middle East and possibly Africa, a shift from its historically eastern-facing posture. They aim to win support to set up a "wise persons" group to draw up reform plans.
In a final communique, NATO allies will recommit to their pledge to defend each other. Britain is expected to put six warships, two fighter squadrons and thousands of troops at NATO's disposal to meet a U.S. demand for European armies to be more combat-ready.
NATO will also warn China for the first time that it is monitoring Beijing's growing military might. Leaders will agree to prepare for conflicts in space, the Arctic and computer networks, as well as traditional land, sea and air battles.
NATO Secretary-General Jens Stoltenberg told diplomats ahead of the gathering that even though disputes were making headlines, the alliance was flourishing.
"I'm a politician, and I'm used to being criticised for good rhetoric but bad substance," Stoltenberg said. "In the case of NATO it is the opposite. We have had bad rhetoric but extremely good substance." (Additional reporting by Andreas Rinke, John Chalmers and Johnny Cotton in Watford, and Estelle Shirbon in London; Writing by Luke Baker; Editing by John Chalmers and Peter Graff)
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