"These disagreements are real. It is not written in stone that the transatlantic bond will survive forever. But I believe we will preserve it," Stoltenberg said in a speech in London.

He noted different views between European nations and US President Donald Trump on the Iran nuclear deal, the environment and trade, which exploded at an acrimonious G7 meeting earlier this month.

But he said he was "absolutely confident" that the US-led military alliance would be able to demonstrate its unity at a summit in July, promising "more cash and capabilities".

"There are many different ties that bind Europe and North America together. We may have seen the weakening of some of them lately," he said.

"But our ties on defence have grown stronger. Since coming to office, the Trump administration has increased funding for the US presence in Europe by 40 percent."

At next month's NATO summit in Brussels, Stoltenberg said he expected Trump to press the case for increased European spending on defence, which he said was rising.

"I expect the president to be very strong on defence spending, I thanked him for his leadership on defence because it has had an impact," he added.

Asked about media reports that Trump would meet Russian President Vladimir Putin during his visit to Europe, he said it would not "in any way" contradict NATO policy.

"We don't want a new Cold War, we don't want a new arms race, we want to talk to Russia, Russia is there to stay," Stoltenberg said.

"We strive for a better relationship with Russia, and even if a better relationship is not possible, we need to talk to them to manage a difficult relationship," he added.

The secretary general was to meet British Prime Minister Theresa May in London, and he welcomed Britain's "leadership" role in NATO.

He said he expected his hosts to "continue and to maintain that role" through further defence spending.