Yet another firestorm is raging in Washington, after US President Trump declassified an explosive memo alleging the Russia investigation was politically motivated from the start.

The four-page document, written by Republicans on the House of Representatives intelligence committee, alleges that the federal probe of potential collusion between Trump's 2016 presidential campaign and Russia was a product of political bias against Trump at the FBI and Justice Department.

The memo also claims the FBI and Justice Department abused their powers to spy on Carter Page, a former Trump campaign advisor, starting in October 2016.

Ignoring a plea from the FBI, Trump approved the memo's release on Friday (February 2) without redactions.

Former FBI chief James Comey mocked the memo's release as being of little interest after all, and branded the document "dishonest and misleading".

What is the memo all about?

The memo alleges that in order to get a court to approve electronic surveillance on Page, the FBI relied on a source who was strongly biased against Trump, former British spy Christopher Steele, and whose dossier of alleged Trump-Russia contacts was funded in part by US Democrats.

The memo said the initial application and subsequent renewal applications requesting surveillance on Page, and signed off by various senior Justice Department officials, did not mention the link between Steele and the Democrats.

The memo revealed the names of senior FBI and Justice Department officials, including Rosenstein, who it said had signed off on this.

The memo was commissioned by the Republican chairman of the House intelligence panel, Devin Nunes. He said it laid bare "serious violations of the public trust, and the American people have a right to know when officials in crucial institutions are abusing their authority for political purposes."

Democrats said the memo mischaracterises highly sensitive classified information and aims to undermine Special Counsel Robert Mueller's criminal probe on the Russia matter, which was launched in May 2017 and grew out of an earlier FBI investigation.

They have warned Trump against using the memo as a pretext to fire any more of the top officials overseeing the Russia probe – such as Deputy Attorney General Rod Rosenstein, who hired Mueller and oversees the investigation, or Mueller himself.

What Trump has said

"I think it's a disgrace what's happening in our country," Trump told reporters when asked about the memo, adding that "a lot of people should be ashamed of themselves."

Asked by reporters whether the memo made him more likely to fire Rosenstein or whether he had confidence in him, Trump replied, "You figure that one out."

A White House official later said there have been no discussions about firing Rosenstein.

Dismissing Rosenstein would likely ignite a huge political firestorm, as his firing of FBI chief Comey did last year.

Trump sacked Comey last May as the FBI pursued the Russia probe, leading to Mueller's appointment by Rosenstein.

Trump has consistently denied any collusion with Russia or obstruction of justice, and called the investigation a "witch hunt". Moscow has denied any election meddling.