Trump okays release of controversial Nunnes-Steele memo

US president Donald Trump authorised the release of a controversial Republican memo alleging intelligence abuses, hours after he accused the Justice Department and the Federal Bureau of Investigation of a "pro-Democrat bias", saying they had "politicised" the probe against Republicans.

Officials of the FBI and Justice Department got a warrant to spy on a Trump campaign associate by misleading a surveillance court judge, House Republicans contend in a newly-released memo that Democrats have dismissed as a contrived account intended to protect the president.

White House spokesman Raj Shah said the document has been transmitted to the minority and majority members in the House Intelligence Committee. The document was also sent to House Speaker Paul Ryan's office, he said. The White House requested no redactions, Shah said.

Speaking in the Oval Office, Trump implied the memo revealed political bias at the FBI. He said he believed the purported bias was a "disgrace" and said certain people should be "ashamed of themselves".

The extraordinary decision to release the classified four-page memo with a never-before-used House Intelligence Committee rule would escalate the partisan fight over the investigations into Russian election meddling and possible collusion.

The Republican memo, written under the direction of House Intelligence chairman Devin Nunes, cites ''a troubling breakdown of legal processes established to protect the American people from abuses'' of the foreign intelligence surveillance court.

Democrats assailed the document as mischaracterising and cherry-picking information to make a politically motivated case and warned against using it as a pretext to fire Special Counsel Robert Mueller or Deputy Attorney General Rod Rosenstein. Such a move would be seen as ''an attempt to obstruct justice'' and spark ''a constitutional crisis,'' top House and Senate Democrats wrote in a letter to President Donald Trump.

The Republicans suggest the FBI may not have pursued a surveillance warrant for Carter Page, who was a Trump campaign adviser and had worked earlier as an investment banker in Moscow, without a controversial dossier from the 2016 presidential campaign that was partly funded by Hillary Clinton's campaign. The dossier, written by a former British spy named Christopher Steele, contained unverified allegations that Trump and his campaign had ties to Russia.

The handling of allegations provided by Steele ''raise concerns with the legitimacy and legality of certain'' interactions that the FBI and Justice Department had with the surveillance court, according to the memo. It cites as an example of anti-Trump bias a comment Steele made to then-Associate Deputy Attorney General Bruce Ohr, saying that he ''was desperate that Donald Trump not get elected and was passionate about him not being president''.

Some Trump supporters pounced on the disclosures as a reason to shut down the investigations into the president and his campaign. ''Should be game over,'' Donald Trump Jr tweeted.

The memo's release would also threaten to further fracture the frayed relationship between the president and his Justice Department and intelligence community, both of which opposed the release of the document, which is based on classified intelligence.

The FBI issued a rare public warning on Wednesday that the memo omits key information that could impact its veracity. The memo, spearheaded by Nunes, alleges that the FBI used the opposition research dossier on Trump and Russia written by Steele to secure a FISA surveillance warrant on former Trump foreign policy adviser Carter Page without disclosing that the dossier was funded in part by Democratic sources.

In a statement earlier this week, Nunes said, "It's clear that top officials used unverified information in a court document to fuel a counter-intelligence investigation during an American political campaign."

The memo alleges FBI abused its surveillance tools during its probe into the alleged Russian collusion in the 2016 US presidential election.

Trump's accusation is a rarest of the rare instance as it maligns people he appointed to their roles, including FBI director Christopher Wray, who he nominated after firing former James Comey in May.

"The top leadership and investigators of the FBI and the justice department have politicised the sacred investigative process in favour of Democrats and against Republicans - something which would have been unthinkable just a short time ago," Trump said in a tweet.

"Rank and File are great people!" he added.

In a follow-up tweet, Trump quoted from a recent speech given by American activist and president of conservative watchdog operation Judicial Watch, Tom Fitton, in which he alleged the Democrats attempted to conceal their funding of a dossier that accused Trump of personal ties to the Kremlin.

"You had Hillary Clinton and the Democratic Party try to hide the fact that they gave money to GPS Fusion to create a dossier which was used by their allies in the Obama administration to convince a court misleadingly, by all accounts, to spy on the Trump Team, Tom Fitton," Trump wrote

But Democrats and some former national security officials said the memo didn't demonstrate significant wrongdoing.

''That's it?'' tweeted former FBI Director James Comey, who was fired by Trump last year. ''Dishonest and misleading memo wrecked the House intel committee, destroyed trust with Intelligence Community, damaged relationship with FISA court, and inexcusably exposed classified investigation of an American citizen. For what? DOJ & FBI must keep doing their jobs.''

The president approved Friday's public release of the four-page memo over the FBI's objections. Written by Republican staffers on the House Intelligence Committee and drawn from classified information, it alleges that deception early on tainted the FBI's continuing inquiry into Russian meddling in the 2016 presidential campaign and whether anyone close to Trump colluded in it.

Trump told reporters Friday that ''Congress will do whatever they're going to do'' with the information in the memo. But he added, ''I think it's a disgrace what's happening in our country'' and ''a lot of people should be ashamed - and much worse than that.''