Nato’s Secretary-General Jens Stoltenberg made an unprecedented pitch to Congress for unity amid transatlantic tensions on Wednesday, as he marked 70 years since its founding charter was signed in Washington.

Mr Stoltenberg gave a ringing defense of "the most successful alliance in history," which has often been derided by Donald Trump, the US president.

He used the speech to underscore America’s "enduring support" for the historic alliance and downplayed disagreements with Mr Trump, saying: "Nato has been good for Europe, but Nato has also been good for the United States."

He added: "We have to be frank. Questions are being asked on both sides of the Atlantic about the strength of our partnership. And, yes, there are differences.

"The strength of Nato is that despite our differences, we have always been able to unite around our core task: to defend each other, protect each other, and to keep our people safe".