US President Donald Trump has dismissed as "fake news" reports that an investigation has been launched into a potential "bribery-for-pardon" scheme at the White House.

The US Justice Department is investigating the alleged bribery scheme involving campaign donations to secure a presidential pardon.

The document, which discusses the legality of searching communications and electronic devices of individuals, including attorneys, is highly redacted, with all identifying information blacked out.

But it refers to a "secret lobbying scheme" directed at "senior White House officials" to gain a presidential "pardon or reprieve of sentence" for an unnamed individual.

The scheme, under investigation since at least August, appears to have involved lobbyists and lawyers, a donor to political campaigns, and a man or woman who is or was in prison and is hoping for presidential intervention.

The filing indicates that the lobbyists and lawyers contacted White House officials requesting a presidential pardon or reprieve, citing the "past substantial campaign contributions" and "anticipated future substantial political contributions" from a donor.

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It suggests that the donor is making the offer on behalf of the person seeking clemency.

The document does not indicate when the actions involved took place, and, in the sections not redacted, there is no reference to Mr Trump or his campaign.

But the filing was revealed amid speculation that, with six weeks left in the White House after losing the 3 November election, President Donald Trump is preparing to grant executive clemency to more people, after pardoning his former national security adviser Michael Flynn last Wednesday.

Mr Trump has granted pardons or sentence reductions to several political allies, including campaign consultant Roger Stone, controversial former Arizona sheriff Joe Arpaio and Republican activist Dinesh D'Souza.

The New York Times reported that Mr Trump has discussed granting preemptive pardons to his lawyer, Rudy Giuliani, his three oldest children and his son-in-law Jared Kushner.

There is also public discussion of Mr Trump issuing a pardon for himself, for any crimes he might be charged with related to his time in office - though the legality of that has never been tested.

"A self-pardon would be a fitting abuse to end Trump's presidency. It would also be corrupt, illegitimate, and void," said Democratic congressman Adam Schiff in a tweet.

Meanwhile, US Attorney General Bill Barr was quoted as saying that there was no evidence of any significant fraud in the election that would reverse Democrat Joe Biden's defeat of Mr Trump.

"To date, we have not seen fraud on a scale that could have affected a different outcome in the election," Mr Barr told the Associated Press in an interview.

Trump threatens military spending veto in social media bias battle

Mr Trump has threatened to veto a major military funding bill unless Congress abolishes a liability law protecting social media firms regularly accused of bias by the president.

Section 230 of the Communications Decency Act gives immunity to tech companies such as Facebook and Twitter from legal action on content posted by users.

Both platforms have found themselves the target of incandescent fury from Mr Trump in recent weeks after they began attaching disclaimers to social media posts by the president that claimed he had lost last month's election due to voter fraud.

Mr Trump has doubled on a months-old push to abolish the statute in response - a move that has been backed by his congressional allies. 

"Section 230... represents a serious threat to our national security and the integrity of the elections," the president tweeted last night.

"Therefore, if the very dangerous & unfair Section 230 is not completely terminated as part of the National Defense Authorization Act (NDAA), I will be forced to unequivocally VETO the Bill," he added, referring to the annual bill that authorises the Pentagon's budget. 

Some of Silicon Valley's biggest companies have found themselves under fire from both sides of the political spectrum for their handling of content during a bitter US presidential campaign this year.

Republican officials have accused tech firms of censoring conservative voices and grilled Facebook chief Mark Zuckerberg and Twitter CEO Jack Dorsey during congressional hearings earlier in November.

Both platforms limited the reach of many of Mr Trump's posts, notably those in which the president rejected his election defeat or questioned the integrity of the voting process.

President-elect Joe Biden has also said Section 230 should be "revoked" but has offered no details of any plan to reform the legislation.