“Europe has got to remember why we have been on this 70-year journey, which nobody should devalue.”

Kerry reminded the audience of the reason why the European Union was set up.

“Europe needs to recognise that the reason people came together was not just economics. It was to stop Europeans from killing Europeans. It was to stop the carnage that came out of World War I and World War II.”

“It's worked, folks,” he added, noting that standard of living has gone up “remarkably”, and security and trade have increased.

“It's been the most – literally on the face of the planet, no region, no assembly of countries has grown as significantly and as powerfully as Europe has.”

His comments came two days after publication of an interview of incoming president Donald Trump, in which he said the United Kingdom was “smart” to choose to leave the European Union.

Obama's successor said he believed other EU members will also leave, and noted that the transatlantic military alliance Nato was “obsolete”.

The Trump interview caused “astonishment and agitation” in Europe, according to Kerry's German counterpart, Frank-Walter Steinmeier.

French foreign minister Jean-Marc Ayrault had added that the best response to the Trump interview “is European unity”.

Kerry seemed to agree.

“You have got to see the strength in unity. I think there is strength in unity,” he said.

America's top diplomat during president Barack Obama's second term in office said his EU counterparts were “friends”.

“I loved working with my 28 colleagues in the EU who are all thoughtful and engaged folks,” said Kerry.