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".

"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.

"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.

"The last US Main Battle Tank left Europe in 2013. But now they're back. With a whole new armoured brigade."

He added that faced with international terrorism, the proliferation of weapons of mass destruction, cyberattacks on "a more assertive Russia", north America and Europe must stand together.

"It is -- and has always been -- in our fundamental interest to stand together," he said.