Nato member states sought to put up a united front on Wednesday as tensions came to a head at the alliance's 70th anniversary meet in London.

US President Donald Trump called Canada's Prime Minister Justin Trudeau "two-faced" after footage emerged of him and other world leaders apparently joking about Trump's lengthy media appearances.

The conversation took place on Tuesday evening at a reception hosted in Buckingham Palace.

Taking a dig at Trudeau for Canada's contribution to the Nato budget not reaching the suggested level of two percent of gross domestic product, Trump said: "He's two-faced. I find him to be a very nice guy, but you know the truth is that I called him out on the fact that he's not paying two percent and I can see he's not very happy about it."

"He's not paying two percent and he should be paying two percent. Canada - they have money," he added.

The US president also cancelled a post-summit press conference and is returning to Washington, after the three-hour meeting with fellow alliance leaders at Watford, north of London.

Trudeau later said that it was not Trump who was the topic of the banter caught on camera, but the location of the upcoming G7 summit, Camp David.

"I have a very good relationship with Trump," he added.

Nato Secretary General Jens Stoltenberg said that the 29-member military alliance remains the only place where Europe and North America discuss, decide and act together every day.

"We stand together all for one, and one for all. Our commitment to Article 5, the collective defense clause of our alliance, is ironclad."

^{CGTN Europe's Phil Lavelle's guide to this week's Nato summit}

British Prime Minister Boris Johnson reiterated the coalition's resolve to engage with each other and praised the US for being a "massive contributor" to Nato and a "pillar of stability for our collective security."

Brushing aside the divisions, Johnson described the alliance as a "giant shield of solidarity".

In a separate briefing, French President Emmanuel Macron told reporters that Russia "can be a threat, but also a partner."

"Not everyone around the Nato table would say Russia was an enemy," he added.

Macron had earlier stated that member-states should focus more on terrorist threats than Russia.

Nato has been beset with a series of rows over future strategies and budget in recent weeks.

France, the US and Turkey traded barbs on Tuesday, with Trump berating Macron on his Nato 'brain dead' remark, last month.

Trump had termed Macron's comments as "very insulting" and a "very dangerous statement" to make.

"Nobody needs Nato more than France," he added.

Macron, in a controversial interview, told The Economist magazine last month: "What we are currently experiencing is the brain death of Nato."

The 41-year-old warned that the coalition was standing on "the edge of a precipice" and should start thinking strategically as a geopolitical power.

When the two men came face-to-face in front of the media on Tuesday the French president said that he stood by his comments.

Macron had also criticized fellow member Turkey for sending its army into northern Syria to fight Kurdish rebels and opting for the Russian S-400 missile defense system, ignoring Nato's concerns.

Turkish President Recep Tayyip Erdogan, showed no signs of backing down, saying that he would block a plan to defend Baltic countries unless the alliance labels the Syrian Kurdish People's Protection Units (YPG) as terrorists.

The military coalition's budget was the subject of discussion, too, as Trump voiced his displeasure at the sums contributed by some states.

"When I came in, I was angry at Nato, and now I've raised $130 billion," Trump said as he referred to the figure Stoltenberg promised European members and Canada would contribute by next year.

"And yet you still have many delinquent - you know I call them delinquent when they're not paid up in full," he added.