North Atlantic Treaty Organization Secretary-General Jens Stoltenberg told CNBC in London on Monday that the alliance needs to address the challenges and opportunities posed by an increasingly powerful China, but added that his 29-member defense organization does not want to make an enemy out of Beijing.

He insisted that the military alliance did not want to "create new adversaries".

"As long as NATO allies stand together, we are strong and we are safe. ... We are by far the strongest military power in the world," he said.

"There's no way that NATO will move into the South China Sea, but we have to address the fact that China is coming closer to us, investing heavily in infrastructure," he added.

Stoltenberg spoke as leaders of the military alliance held their 70th anniversary summit north of London, where a tense opening on Tuesday revealed divisions on key issues.

Discussions are also expected to focus on the growing threat of cyberattacks and security in space, but cracks emerged within the group as United States President Donald Trump and French President Emmanuel Macron set out opposing views.

The 29-member bloc's future is not in doubt, but there were sharp exchanges over many topics on Tuesday as the French president accused Turkey of colluding with Islamic State proxies while Trump described Macron's criticisms of NATO's "brain death" as insulting and "very, very nasty".

The Turkish president, Recep Tayyip Erdogan, has threatened to veto NATO efforts to bolster protection of Baltic countries against Russia unless the allies brand the Kurdish militias who defeated Islamic State in Syria as "terrorists", a definition that Macron and the US reject.

The gathering at a luxury hotel near Watford follows receptions held on Tuesday at Buckingham Palace and Downing Street.

Protesters gathered outside the palace, rallying against NATO and Trump and his perceived interest in the United Kingdom's National Health Service in a US-UK trade deal.

Trump and British Prime Minister Boris Johnson spoke earlier on Tuesday in an unscheduled separate meeting on the summit's sidelines. Trump tweeted that they "talked about numerous subjects including NATO and trade".

According to Downing Street, Johnson, as host, would later tell gathered leaders at the summit: "Seventy years on, we are rock solid in our commitment to NATO and to the giant shield of solidarity that now protects 29 countries and nearly 1 billion people.

"The fact that we live in peace today demonstrates the power of the simple proposition at the heart of this alliance: that for as long as we stand together, no one could hope to defeat us — and therefore no one will start a war."

Macron stood by his comment that NATO was experiencing "brain death", saying on Wednesday it had spurred the right kind of debate about the alliance's direction.

He said: "I think it was our responsibility to raise ambiguities that could be harmful, and to tackle a real strategic debate. It has started, I am satisfied."

Trump said the European allies were "delinquent" in their failure to spend as much as the US on their armed forces. In response, Europeans have said that they will spend $400 billion collectively on defense by 2024.

"If we invest money and risk our soldiers' lives … we should be clear on NATO's fundamentals," Macron said on Twitter, adding that on Wednesday he would "defend the interests of France and Europe".

The three-hour summit was also expected to consider new threats, including in the areas of cyberspace and outer space, after NATO declared last month they were among its operational domains alongside air, land and sea.