US President-elect Joe Biden on Sunday said that his country is facing "four historic crises at once", adding that his team is work working hard to tackle the time-bound challenges.

"From COVID-19 and the economy to climate change and racial justice -- our nation is facing four historic crises at once. And come January, there will be no time to waste. That's why my team and I are hard at work preparing to take action on day one," Joe Biden tweeted.

Hours after Joe Biden's tweet, President Trump officially signed the trillion-dollar coronavirus relief and government spending bill, The Hill reported.

On Saturday, Joe Biden had accused Donald Trump of "abdicating responsibility" and had pressed the outgoing President to immediately sign the COVID-19 relief bill that was pending.

"It is the day after Christmas, and millions of families don't know if they'll be able to make ends meet because of President Donald Trump's refusal to sign an economic relief bill approved by Congress with an overwhelming and bipartisan majority," Joe Biden said in a statement.

Members of both parties had urged Donald Trump to sign the USD 2.3 trillion packages, The Hill reported. Earlier, the bill was passed with bipartisan support in US Congress.

Earlier today, top infectious disease expert Dr Anthony Fauci said the United States is headed for the worst of the COVID-19 pandemic in the upcoming weeks as Americans will see the effects of the holiday season.

"The reason I'm concerned and my colleagues in public health are concerned is that we very well might see a post-seasonal, in the sense of Christmas, New Year's, surge," CNN quoted Dr Anthony Fauci as saying.

"We are really at a very critical point. So I share the concern of President-elect Joe Biden that as we get into the next few weeks, it might actually get worse," Dr Anthony Fauci added.

Joe Biden last week had said that "darkest days" against coronavirus "are ahead of us, not behind us."

The COVID-19 case total in the United States surpassed the 19 million mark on Sunday (local time), according to the US-based Johns Hopkins University, which tracks and compiles data from various sources including local authorities and media outlets.

As of 6:35 IST on Monday, Johns Hopkins University had registered 19,107,675 positive tests for the coronavirus disease in the United States. The country's deaths, at the same time, stood at 333,069.