(Adds White House working to release document, Thune and Pelosi comments)
WASHINGTON, Feb 1 (Reuters) - The White House was working on Thursday to clear the release of a secret Republican memo alleging FBI bias against President Donald Trump, an administration official said, disregarding a warning from the top U.S. law enforcement agency.
As White House legal and national security experts reviewed the document, the Senate's No. 3 Republican, John Thune, urged his House of Representatives colleagues to allow the Senate Intelligence Committee to review the document and to heed the FBI's concerns before they made it public.
Democrats have called the four-page memo misleading, based on a selective use of highly classified data and intended to discredit Special Counsel Robert Mueller's criminal probe into potential collusion between the Trump campaign and Russia to help him win the 2016 presidential election.
The document was crafted by Republicans on the House Intelligence Committee, and commissioned by Devin Nunes, the panel's Republican chairman.
Top House Democrat Nancy Pelosi, on Thursday sent a letter to Republican House Speaker Paul Ryan calling for Nunes to be removed as chairman, saying Nunes had acted dishonestly in altering the memo. The committee's senior Democrat, Adam Schiff, late on Wednesday said he had discovered Nunes had sent the White House a version of the memo that was "materially altered" and not what the panel voted on Monday to release.
"This action is not only dangerous, it is illegitimate, and violates House rules," Pelosi said.
Mueller's investigation and the FBI probe that preceded it have hung over Trump's year-old presidency. Russia and Trump have both denied any collusion.
The Federal Bureau of Investigation issued a rare rebuke on Wednesday to the president and his fellow Republicans in Congress who are pushing to release the memo.
"The FBI was provided a limited opportunity to review this memo the day before the committee voted to release it," the FBI said in a statement. "As expressed during our initial review, we have grave concerns about material omissions of fact that fundamentally impact the memo's accuracy."
Justice Department officials have also said releasing the memo could jeopardize classified information.
The administration official told Reuters earlier the memo was likely to be released on Thursday. The timing, however, remained in flux.
Nunes has dismissed the objections to the memo's release as "spurious."
Thune, attending a political retreat in West Virginia with other lawmakers in Trump's party, urged fellow Republicans to slow down regarding the memo's release.
"I think the Senate Intelligence Committee needs to see it, for sure," Thune said. "They need to pay careful attention to what our folks who protect us have to say about what this, you know, how this bears on our national security."
Thune also said he believed that a counterpoint memo written by Democrats on the House Intelligence Committee should be made public if the Republican document is released.
The Republican memo accuses the FBI and Justice Department of misleading a Foreign Intelligence Surveillance Court judge in March as they sought to extend an eavesdropping warrant against Trump campaign adviser Carter Page, four sources familiar with it have said.
They said memo contends that the FBI and Justice Department failed to tell the judge that some of the information used to justify the warrant included portions of a dossier of Trump-Russia contacts that was opposition research paid for by Democrats.
However, the sources said the memo does not mention that the request to extend surveillance on Page, which began before Trump took office, also relied on other highly classified information and that U.S. agencies had confirmed excerpts from the dossier included in the request.
U.S. intelligence agencies have concluded that Russia interfered in the 2016 campaign using hacking and propaganda to attempt to tilt the race in favor of Trump. The president has called Mueller's investigation a "witch hunt" and "hoax."
(Additional reporting by Amanda Becker, Karen Freifeld, Jonathan Landay, Doina Chiacu; Editing by Will Dunham)