The head of the FBI has set up an extraordinary public showdown with President Donald Trump, publicly saying there were "grave concerns" about the accuracy of a classified memo on the Russia investigation that Republicans want to release to the public.

It was the first time the bureau has explicitly weighed in publicly on an issue that has put Mr Trump and senior Republicans in conflict with the Justice Department.

The memo was written by Republicans on the House intelligence committee under the leadership of congressman Devin Nunes, a backer of Mr Trump.

Republicans have said the memo reveals improper use of surveillance by the FBI and the Justice Department during the early stages of the investigation into whether there were links between Mr Trumps' campaign and Russia. The memo was said to reveal political bias in the investigation.

FBI Director Christopher Wray reportedly visited the White House and told chief of staff John Kelly the memo contains inaccurate information and offers a false picture.

In a public statement the bureau said: "The FBI was provided a limited opportunity to review this memo the day before the committee voted to release it.

"As expressed during our initial review, we have grave concerns about material omissions of fact that fundamentally impact the memo’s accuracy."