WASHINGTON (REUTERS) - US President Donald Trump attacked the integrity of the FBI and Justice Department on Friday (Feb 2) in an extraordinary showdown over the White House's expected approval for the release of a secret Republican memo alleging FBI bias against him in its Russia probe.

Trump, who has long been angered over the probe, accused the country's top law enforcement officers - some of whom he appointed himself - of politicising investigations in favour of Democrats and against his fellow Republicans.

Trump has repeatedly complained about his treatment by federal investigators who are probing possible collusion between his campaign and Russia to sway the 2016 presidential election, and any actions to impede the investigation.

"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 on Twitter.

The president praised "rank and file" FBI employees.

His latest salvo was sure to worsen the president's frayed relations with agencies that are supposed to be politically independent. Two days ago, in a rare public rebuke of the president and Republicans in Congress who are pushing to release the classified memo, the FBI said it had "grave concerns about material omissions of fact" in the document and it should not be made public.

The Justice Department had no comment on the president's attack, spokeswoman Sarah Isgur Flores said. The Federal Bureau of Investigation did not respond to a request for comment.

A White House official said Trump was likely to give Congress approval on Friday to release the document, which has become a flashpoint in a wider battle between Republicans and Democrats over Special Counsel Robert Mueller's Russia probe.

In an effort to defuse the conflict, Republican House Speaker Paul Ryan backed the release of a Democratic counterpoint memo if the classified Republican document were made public. Ryan can block either but has supported releasing the Republican memo.

The four-page memo was commissioned by the Republican chairman of the House Intelligence Committee, Devin Nunes. It purports to show that the FBI and Justice Department misled a US court in seeking to extend electronic surveillance of Trump campaign adviser Carter Page, multiple sources familiar with it have said.

Democrats have depicted the memo, which was crafted by Republican members of the intelligence panel, as misleading, based on a selective use of highly classified data and intended to discredit Mueller's work.

Mueller's criminal probe grew out of the FBI's Russia investigation after Trump fired FBI Director James Comey and Attorney-General Jeff Sessions recused himself from the investigation. Russia has denied meddling in the election campaign. Trump, calling Mueller's probe a political witch hunt, has denied collusion or obstruction of justice.

Democrats say their counter-memo restores context and information left out of the Republican version. Republicans have resisted releasing that document, but Ryan's office said on Friday he backed making the Democrats' rebuttal public if it does not reveal intelligence gathering sources or methods.

In the Senate, some Republicans including John Thune and Lindsey Graham have opposed the classified memo's release.

REBUKES FOR TRUMP

US Representative Adam Schiff, the top Democrat on the House intelligence panel, responded to Trump's tweets with his own Twitter message. "The country's top elected leader has agreed to selectively and misleadingly release classified info to attack the FBI - that's what would have been unthinkable a short time ago," he said.

There has been speculation that FBI director Christopher Wray, appointed by Trump after he fired Comey, might resign if Trump allows release of the memo.

Trump has openly criticised Attorney General Jeff Sessions for recusing himself from overseeing the Russia probe last March following media reports that he had failed to disclose meetings in 2016 with Moscow's ambassador at the time, Sergei Kislyak.

The deputy attorney-general appointed to lead the probe when Sessions stepped aside from the issue, Rod Rosenstein, has also drawn Trump's ire, most recently over his role in the surveillance warrant at the heart of the Republican memo.

James Clapper, the former director of national intelligence, said Trump's attack on the FBI and Justice Department was the"pot calling the kettle black."

Republican efforts to release selective portions of classified information in the memo was a "blatant" political act, he told CNN.

Although Trump's tweet charged a bias in favour of Democrats, many of the people it targets are Republicans: Wray, Sessions and Rosenstein. Many Democrats blamed Comey, also a Republican, for Democratic candidate Hillary Clinton's election defeat in 2016 after he announced the discovery of additional emails from her private server just 11 days before the vote.