President-elect Joe Biden has downplayed concerns that Donald Trump’s refusal to acknowledge his victory could undermine national security.

Raising unsupported claims of voter fraud, President Trump has blocked his Democratic rival from receiving the intelligence briefings traditionally shared with incoming presidents, according to someone with knowledge of the situation but not authorized to disclose private conversations.

President Trump’s resistance, backed by senior Republicans in Washington and across the country, could also prevent background investigations and security clearances for Mr Biden’s prospective national security team and access to federal agencies to discuss budget and policy issues.

Mr Biden downplayed the impact of the Republican resistance, which he said “does not change the dynamic at all in what we’re able to do”.

Additional intelligence briefings “would be useful”, Mr Biden said, but “we don’t see anything slowing us down, quite frankly”.

Despite growing frustration, Mr Biden delivered an afternoon speech on the Affordable Care Act, just hours after the Supreme Court heard arguments on its merits.

The high court ruled eight years ago to leave intact the essential components of the law known as “Obamacare”, but President Trump and his Republican allies are seeking to have it overturned.

If the 6-3 conservative court ultimately agrees with the GOP, millions of Americans could lose their health care coverage.

While Tuesday’s arguments indicate the court is unlikely to strike down the entire law, the prospect adds to the pressure on Mr Biden to execute complicated plans to confront a series of crisis he will inherit in just 71 days.

The nation’s economy is struggling as the pandemic surges and cultural divisions deepen.

President Trump and his allies seem determined to make Mr Biden’s transition as difficult as possible.

From his Twitter account on Tuesday, President Trump again raised unsupported claims of “massive ballot counting abuse” and predicted he would ultimately win the race he has already lost.

His allies on Capitol Hill, led by Senate Majority Leader Mitch McConnell, have encouraged the president’s baseless accusations.

President Trump’s tweets were swiftly flagged by the social media network as claims about election fraud that are disputed.

Mr Biden tried to stay focused on health care as he prepares to take office on January 20 during the worst health crisis in more than a century.

The US surpassed 10 million cases on Monday and cases are skyrocketing as the nation moves into the cold winter months.

Attorney General William Barr has authorised the Justice Department to probe unsubstantiated allegations of voter fraud.

And the General Services Administration, led by a Trump-appointed administrator, Emily Murphy, has declined to formally recognise Mr Biden as president-elect.

That designation eases cooperation between the outgoing and incoming administrations, although Ms Murphy has not started the process and has given no guidance on when she will.

The GSA inaction could continue to deny Mr Biden security briefings, which he received periodically before the election, as well as delaying security clearances and staffing decisions.

Senior officials in the George W Bush administration warned that a similar delay after the closely contested 2000 presidential election caused many difficulties.

Republicans are increasingly eyeing a December deadline to give President Trump time and space to exhaust his legal challenges.

That is when the states face a deadline to certify results and a December 14 deadline for the Electoral College to cast its votes.

It is also about the time it took to resolve the 2000 election dispute between Republican George W Bush and Democrat Al Gore.

“Until the Electoral College votes, anyone who is running for office can exhaust concerns,” Mr McConnell told reporters on Capitol Hill.