Monday's development came as President Donald Trump has refused to concede to his Democratic rival and incumbent President-elect Joe Biden, who was projected the winner of the November 3 election after crossing the threshold of 270 electoral college votes needed to secure victory.

In a memo on Monday, Barr authorised federal prosecutors "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 inspecific instances".

"Such inquiries and reviews may be conducted if there are clear and apparently credible allegations of irregularities that, if true, could potentially impact the outcome of a federal election in an individual state," he added.

While acknowledging it was imperative that "credible allegations be addressed in a timely and effective manner", Barr noted that it was "equally imperative that Department personnel exercise appropriate caution and maintain absolute commitment to fairness, neutrality and non-partisanship".

"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," he added.

Barr's announcement prompted the resignation of senior Justice Department official Richard Pilger, who would have overseen such investigations, the BBC reported.

"Having familiarised myself with the new policy and its ramifications... I must regretfully resign from my role," he was quoted as saying in an email to colleagues just hours after the announcement.

Also on Monday, the Trump re-election campaign filed a lawsuit in a Pennsylvania federal court, seeking an emergency injunction to stop state officials from certifying Biden's victory in the key swing state.

But the state's Attorney General, Josh Shapiro, called the lawsuit "meritless".

Meanwhile, atates must resolve recounts and court contests over the final outcome by December 8.

The outcome will be finalised when members of the US Electoral College meet on December 14.