President Donald Trump told the Russians that the former FBI director was a “nut job” as a way of reassuring them that the consternation about Moscow’s influence would die down, a senior White House official said on Sunday.

H.R. McMaster, the national security adviser, said that Mr Trump’s May 10 comments to Sergei Lavrov, the foreign minister, and Sergey Kislyak, the ambassador, were intended as a sweetener.

"The gist of the conversation was that the president feels as if he is hamstrung in his ability to work with Russia to find areas of cooperation because this has been obviously so much in the news," said Mr McMaster.

The "nut job" conversation, reported on Friday night, set off yet another firestorm in Washington. The US president reportedly said: “I just fired the head of the FBI. He was crazy, a real nut job. I faced great pressure because of Russia. That’s taken off.” His comments increased fears that Mr Trump obstructed justice, firing James Comey in a bid to scotch the Russia investigation.