As the 70th-anniversary summit of NATO began amid divisions on Tuesday, the alliance's Secretary-General Jens Stoltenberg said he was working to solve a dispute with Turkey over allied plans to defend the Baltics.
Asked if the issue could be resolved by the end of the London summit, he said: "I will not promise that, but what I can say is that we are working on that. But it is not like NATO doesn't have a plan to defend the Baltic countries."
Speaking after breakfast with US President Donald Trump, Stoltenberg said NATO still had strong support in the United States and cited a "big paradox" that while people were questioning the transatlantic bond, citizens backed NATO.
"We have bad rhetoric but extremely good substance," he said.
Trump bashes European allies
Earlier, Trump bashed his European allies at a joint press conference with Stoltenberg.
"The European Union (is) treating the United States very, very unfairly on trade," Trump said.
"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."
"We're talking to the European Union about trade and they have to shape up or otherwise things are going to get very tough."
Trump criticised his French counterpart Emmanuel Macron for saying last month that the alliance was "brain dead." It is a "very insulting statement to make," he said.
"It's a tough statement, though, when you make a statement like that, that is a very, very nasty statement to essentially 28, including them, 28 countries," Trump told reporters.
"I think that, you know, you have a very high unemployment rate in France. France is not doing well economically at all. They're starting to tax other people's products so therefore we go and tax them."
Later in the press conference, Trump said he saw France as breaking off from NATO even though no one needed the alliance more than Paris.
What's on the agenda?
NATO leaders are expected to sign off on a set of decisions already taken by NATO foreign and defence ministers. These include making space a full domain of conflict -- alongside land, sea, air and cyber space -- as well as a new report on how the alliance should approach China and its growing international assertiveness.
World leaders will be hosted at Buckingham Palace on Tuesday by Queen Elizabeth, and have the news conference on Wednesday in Hertfordshire, in England.
But the most sensitive matters may not be on the official agenda. Here are five things to look out for in the summit.
'Burden sharing' spat
Dismissing recent signals from Germany that it was ready to do more to match a NATO target of spending two per cent of national output on defence, Trump on Tuesday accused it and other nations which spend less than that target of being "delinquent".
Trump has repeatedly lambasted his European allies for what he considers freeloading on America for their defence.
NATO officials had hoped a managed series of spending announcements by Secretary-General Jens Stoltenberg last week would appease US concerns.
"This has been the fifth consecutive year of rising defence investment, with European Allies and Canada due to spend $130 billion extra by the end of 2020, with that figure rising to $400 billion by the end of 2024," NATO said on its website.
En route, Trump took credit for the rise on Twitter: "Since I took office, the number of NATO allies fulfilling their obligations more than DOUBLED."
'Brain death' row
French President Emmanuel Macron raised eyebrows across Europe last month after suggesting the Alliance is experiencing "brain death" in the current geopolitical climate. His comments will likely overshadow the summit.
Germany, the US and even NATO itself have rejected Macron's criticisms — Angela Merkel called them "drastic.
In response, Germany has suggested setting up an expert panel to look at how NATO can be adapted to address political questions more effectively.
On Friday Turkey's President Recep Tayyip Erdogan, furious at Western criticism of his military operation in northern Syria against the Kurds, hit back with a personal attack on Macron.
"First of all, have your own brain death checked. These statements are suitable only to people like you who are in a state of brain death," Erdogan declared, insisting he would say the same again at NATO.
French officials summoned the Turkish envoy in Paris to complain while a US administration official predicted that many members would tackle Turkey over its purchase of a Russian S-400 air defence system.
Macron and Erdogan will come face to face on Tuesday in a four-way meeting with Merkel and British Prime Minister Boris Johnson.
How to handle Russia
Hoe to deal with Russia is another divisive issue for NATO members.
In recent weeks, Macron has advocated for dialogue with Moscow, drawing concerns from eastern European countries.
“We need to reopen a strategic dialogue, without being naive and which will take time, with Russia,” the French President said last month.
Outgoing European Council President Donald Tusk dismissed the idea, calling Russia “aggressive” and bent on undermining Europe.
On Tuesday ahead of the summit, Stoltenberg took a strong stance against Russia.
"Through the presence of NATO forces in Poland and in the Baltic countries, we are sending Russia a very strong signal: if there is an attack on Poland or the Baltic countries, the whole alliance will respond," he told a group of newspapers, including Polish daily Rzeczpospolita, Germany's Suddeutsche Zeitung and Spain's El Pais.
The United States has urged Turkey to give its support to a NATO defence plan for the Baltics and Poland. Turkey has refused so far, saying NATO should give Ankara more political backing in its fight against Kurdish militia in Syria.
The summit also comes as Britain is in the midst of a frenetic election campaign.
With Johnson leading polls, the prime minister seeks to avoid at all costs that Trump wades into the December 12 election.
But moments after claiming he did not want to get involved in the British election campaign, Trump said on Tuesday that he thought Prime Minister Boris Johnson was "very capable" and would do a good job.
Trump has featured heavily in the British election campaign, where his name is used by the Labour Party as a by-word for a capitalist system that Corbyn, a veteran socialist campaigner, has promised to tear up if he wins power.
The NATO summit takes place as Trump battles an effort led by Democrats who control the US House of Representatives to force his removal from office through impeachment over his pressure on Ukraine to investigate Democratic rival Joe Biden.
The impeachment imbroglio has overshadowed Trump's presidency as he looks ahead to his own re-election fight next November.
Asked about the inquiry at a press conference on Tuesday, Trump said:"The impeachment thing is a hoax, it has turned out to be a hoax. It is done for purely political gain, they are going to see whether or not they can do something in 2020 because otherwise,they are going to lose."