US Attorney General William Barr has assigned a senior federal prosecutor to examine the origins of the Russia investigation.

John Durham, the US attorney in Connecticut, has reportedly been asked to determine whether the collection of intelligence on the Trump campaign was lawful.

President Donald Trump has long called for such an investigation.

He has branded the Russia inquiry a “witch hunt” by his opponents.

Critics of Mr Barr, however, have accused him of acting on behalf of the president rather than in the interests of US justice.

Speaking to reporters outside the White House on Tuesday, Mr Trump said he had not directed Mr Barr to investigate the origins of the Russia probe.

“I didn’t ask him to do that… But I think it’s a great thing that he did,” Mr Trump said. “I am so proud of our attorney general.”

Last month, Mr Barr told members of Congress that he believed “spying did occur” on the Trump campaign in 2016, adding: “The question is whether it was adequately predicated. And I’m not suggesting that it wasn’t adequately predicated. But I need to explore that.”

He gave no details but said he was gathering a team to look into the origins of the investigation, which was led by special counsel Robert Mueller.