Barr's directive to prosecutors prompted the top lawyer overseeing voter fraud investigations to resign in protest. It came after days of attacks on the integrity of the election by Trump and Republican allies, who have alleged widespread voter fraud, without providing evidence.

Trump has not conceded the election to Democrat Joe Biden, who on Saturday secured the more than the 270 votes in the Electoral College needed to win the presidency.

The Trump campaign has filed several lawsuits claiming the election results were flawed. Judges have tossed out lawsuits in Michigan and Georgia, and experts say Trump's legal efforts have little chance of changing the election result.

Barr told prosecutors on Monday that "fanciful or far-fetched claims" should not be a basis for investigation and his letter did not indicate the Justice Department had uncovered voting irregularities affecting the outcome of the election.

But he did say he was authorizing prosecutors to "pursue substantial allegations" of irregularities of voting and the counting of ballots.

Richard Pilger, who for years has served as director of the Election Crimes Branch, announced in an internal email he was resigning from his post after he read "the new policy and its ramifications".

The previous Justice Department policy, designed to avoid interjecting the federal government into election campaigns, had discouraged overt investigations "until the election in question has been concluded, its results certified, and all recounts and election contests concluded".

Biden's campaign said Barr was fueling Trump's far-fetched allegations of fraud.

"Those are the very kind of claims that the president and his lawyers are making unsuccessfully every day, as their lawsuits are laughed out of one court after another," said Bob Bauer, a senior adviser to Biden.

Earlier on Monday, Trump's campaign filed a lawsuit to block Pennsylvania officials from certifying Biden's victory in the battleground state, where Biden led by more than 45,000 votes.

It alleged the state's mail-in voting system violated the US Constitution by creating "an illegal two-tiered voting system" where voting in person was subject to more oversight than voting by mail.

It was filed against Pennsylvania Secretary of State Kathy Boockvar and the boards of elections in Democratic-leaning counties that include Philadelphia and Pittsburgh. Boockvar's office did not immediately respond to a request for comment.

"The Trump campaign's latest filing is another attempt to throw out legal votes," Pennsylvania Attorney General Josh Shapiro, a Democrat, said on Twitter. "Sleep tight. We will protect the laws of our Commonwealth and the will of the people."

Jessica Levinson, a professor at Loyola Law School in Los Angeles, said the latest lawsuit in Pennsylvania was unlikely to succeed and "reads like a rehash of many of the arguments the Trump legal team has made in and outside the courtroom".

Trump's re-election team asked for patience on Monday to pursue allegations of voter fraud.

"This election is not over, far from it," Kayleigh McEnany, the White House press secretary, told reporters at a briefing in what she said was her capacity as a Trump campaign adviser.

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