US President Donald Trump has once again picked Republican lawmaker John Ratcliffe to be the nation's top intelligence official.

Trump, who announced the moved on Twitter on Friday, first nominated Ratcliffe, a fierce defender of the president, last year but withdrew the nomination amid questions about Ratcliffe's lack of experience and possible resume embellishment.

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The move comes as Trump faces increasing criticism among Democrats for firing a wave of White House employees from the US national security community, who were perceived as critical of him.

"I am pleased to announce the nomination of @RepRattcliffe (Congressman John Ratcliffe) to be Director National Intelligence," Trump tweeted. "Would have completed the process earlier, but John wanted to wait until after IG Report was finished."

It was unclear to which report Trump was referring. Ratcliffe's office did not immediately respond to a request for comment.

"John is an outstanding man of great talent," Trump tweeted.

His announcement drew a lukewarm response from Richard Burr, the Republican chairman of the Senate Intelligence Committee, who said he looked forward to "receiving Congressman Ratcliffe's official nomination and ushering it through the Senate's regular order".

Democrats objected, with Minority Leader Chuck Schumer calling for swift bipartisan rejection of Ratcliffe as overseer of the 17 agencies comprising the US intelligence community.

"The last time this nomination was unsuccessfully put forward, serious bipartisan questions were raised about Rep. Ratcliffe's background and qualifications," Mark Warner, the top Democrat on the Senate Intelligence Committee, said in a statement. "It's hard for me to see that anything new has happened to change that."

House Speaker Nancy Pelosi said the president was ignoring "many serious outstanding concerns" about Ratcliffe and that intelligence should never be guided by partisanship or politics.

"Unfortunately, Congressman Ratcliffe has shown an unacceptable embrace of conspiracy theories and a clear disrespect and distrust of our law enforcement and intelligence patriots that disqualify him from leading America's intelligence community," Pelosi said in a statement.

Fierce loyalist

Ratcliffe, who has represented a Texas congressional district since 2015 and is a member of the House intelligence and judiciary committees, was an outspoken defender of the president during the Democratic-led proceedings that resulted in Trump's impeachment last year on charges of abuse of power and obstruction of Congress.

The Republican-controlled Senate acquitted Trump this month.

Trump first nominated Ratcliffe to be the director of national intelligence (DNI) on July 28, 2019, to succeed Dan Coats, a former Republican senator with whom Trump clashed over assessments involving Russia, North Korea and Iran.

News outlets, including Reuters, reported on concerns that Ratcliffe exaggerated his counterterrorism experience as a federal prosecutor in Texas.

In a February 25, 2015 press release, Ratcliffe said he had "convicted individuals" in the prosecution of a charity that funnelled money to Hamas, the Palestinian armed group on the US list of foreign "terrorist organisations".

Three defence lawyers said they had no recollection of his involvement in the case.

Trump dropped the nomination on August 2 last year, with Ratcliffe saying that he did not want a partisan "national security and intelligence debate surrounding my nomination, however untrue".

Earlier this month, Trump appointed Richard Grenell, former ambassador to Germany and a staunch Trump loyalist, as the acting DNI, which does not require Senate approval.

The revival of the Texas congressman's nomination lets Grenell remain as acting DNI while the Senate considers Ratcliffe. Grenell was limited to serving until March 11 in a temporary capacity unless Trump tapped a full-time replacement.

The president named Grenell to replace Joseph Mcguire, who also served in an acting capacity, after an aide to the former Navy admiral and intelligence veteran briefed the House Intelligence Committee on Russian attempts to interfere in the 2020 presidential race.