WASHINGTON (REUTERS) - US Attorney-General William Barr told federal prosecutors on Monday (Nov 9) to look into "substantial" allegations of voting irregularities but urged them not to pursue "fanciful or far fetched claims."

The letter from Mr Barr followed days of attacks on the integrity and legality of the election by President Donald Trump and his Republican allies, who have alleged without evidence that there was widespread voter fraud.

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

Earlier Monday, Mr Trump campaign filed a lawsuit to block Pennsylvania officials from certifying Mr Biden's state victory.

"I authorise you to pursue substantial allegations of voting and vote tabulation irregularities prior to the certification of elections in your jurisdictions in certain cases, as I have already done in specific instances," Mr Barr wrote in the letter to federal prosecutors and the FBI director.

It is the first time Mr Barr has addressed claims of voter fraud since the Nov 3 election between Mr Trump and Mr Biden.

"While serious allegations should be handled with great care, specious, speculative, fanciful or far-fetched claims should not be a basis for initiating federal inquiries," Mr Barr wrote.

He said nothing in his letter should be read to indicate the Justice Department had in fact uncovered voting irregularities that impacted the outcome of the election.

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

Biden campaign said Mr Barr was fuelling the very far-fetched claims he claimed he was guarding against.

"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 Mr Bob Bauer, a senior adviser to Mr Biden.

On Monday, the Trump campaign and two registered voters brought a lawsuit in federal court alleging Pennsylvania'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.

Ms Boockvar's office did not immediately respond to a request for comment.

The Trump campaign has filed several lawsuits since claiming the election results were flawed.

Judges have tossed lawsuits in Michigan and Georgia, and experts say the Trump legal efforts have little chance of changing the outcome of the election.

Professor Jessica Levinson from Loyola Law School in Los Angeles said the latest lawsuit 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."

Also on Monday, some Republican state legislators in Pennsylvania said they would "call for a legislative-led audit of the 2020 election and demand election results not be certified, nor electors be seated, until the audit is complete."

Pennsylvania House Speaker Bryan Cutler, a Republican, has said the state's governor certifies the election results and the legislature plays no formal role.

"The popular vote determines the electors, and it's the governor who certifies the vote, so lawmakers have no role to play," said Mr Jason Gottesman, a spokesman for Mr Cutler.

In the United States, a candidate becomes president by securing the most "electoral" votes rather than winning a majority of the national popular vote.

Electors generally cast their vote for the winner of the popular vote in their respective states. They are slated to meet on Dec 14.

The Pennsylvania case was assigned to US District Judge Matthew Brann, an appointee of former President Barack Obama.

A senior Trump campaign official said the campaign had collected "hundreds" of affidavits from Pennsylvania voters alleging election violations.

The campaign is gathering evidence to help make the case that the state's election was fatally flawed, the official said.

Mr Barr's letter came several hours after he met with Republican Senate Majority Leader Mitch McConnell, who earlier on Monday said Mr Trump was well within his rights to look into claims of "irregularities" in last week's election.

A Justice Department official declined to comment on what Mr Barr and Mr McConnell discussed, but told reporters that no one at the White House or on Capitol Hill asked him to write the letter.

Prof Justin Levitt from Loyola Law School who previously served as a deputy assistant attorney general in the Civil Rights Division, said the memo appeared to give "cover" for US attorneys to go out and publicly announce investigations.

"It's very unusual. The Justice Department doesn't announce the start of investigations," Prof Levitt said.