US Attorney General William Barr has approved an investigation into claims of voter fraud.

Donald Trump has so far refused to concede defeat in the US election.

US Attorney General William Barr gave blanket authorization on Monday for federal prosecutors to open investigations into alleged voting irregularities in the US presidental election.

Barr reversed federal policy and unleashed prosecutors from pursuing election fraud before all votes were officially tallied.

"Given that voting in our current elections has now concluded, I authorize you to pursue substantial allegations of voting and vote tabulation irregularities prior to the certification of elections in your jurisdictions in certain cases," Barr said in a letter to US attorneys around the country.

"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."

Reversal of policy

Such investigations normally fall under the jurisdiction of individual states, and the usual policy of the Justice Department is to hold back any federal involvement until vote tallies are certified, recounts completed and races concluded.

However, Barr said that "practice has never been a hard and fast rule," and said that if investigators found anything that could reverse the results of the election, they should pursue it.

"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 wrote.

Shortly after Barr sent the letter, US media reported that the head of the Justice Department's Election Crimes Branch, which oversees investigations into voter fraud, resigned.

Branch Director Richard Pilger wrote in an email to his colleagues: "Having familiarized myself with the new policy and its ramifications... I must regretfully resign from my role," The New York Times reported.

Legal challenge

US President Donald Trump has refused to concede defeat in the elections, despite Democratic challenger Joe Biden being roundly declared the clear victor.

Trump continues to claim that he lost the election due to fraud but has not supported this with evidence .

Some fellow Republicans, including Senate Majority Leader Mitch McConnell, have said Trump is "within his rights" to challenge the election results.

"President Trump is 100% within his rights to look at the allegations of irregularities and weigh his legal options," McConnell said on the Senate floor. "I believe the president may have legal challenges under way in at least five states," he added.