Joe Biden has warned “people may die” if his incoming presidential administration continues to be impeded by President Donald Trump.
Speaking in Delaware, the president-elect said co-ordination was needed to tackle the coronavirus outbreak.
He again called Mr Trump’s refusal to acknowledge he lost the election, despite calls to do so from both main political parties, “embarrassing”.
“This is not a game,” former first lady Michelle Obama wrote on social media.
President-elect Biden has 306 votes in the electoral college, surpassing the 270 threshold needed to win.
Yet Mr Trump tweeted on Monday morning: “I won the Election!”
The Trump campaign launched a flurry of legal challenges in the wake of the 3 November election to contest ballot counts, and he has repeatedly taken to Twitter to claim that he “won” the election.
The General Services Administration (GSA), the government agency tasked with beginning the transition process for a new president, has yet to recognize Mr Biden and his running mate Kamala Harris as winners, leaving them without access to sensitive government briefings that are normally provided to an incoming administration.
Aides to the president-elect have said that Mr Trump’s refusal to engage in a presidential transition also means Mr Biden’s team has been excluded from planning around a vaccination distribution strategy.
In his speech on Monday, Mr Biden called the refusal “totally irresponsible”.
“Does anyone understand this?” he said. “It’s about saving lives, for real, this is not hyperbole.”
“More people may die if we don’t co-ordinate,” he said. “If we have to wait until January 20th to start that planning, it puts us behind.”
More than a week after Mr Biden was projected to have won the election, Mr Trump has not conceded.
However, pressure to do so is coming from both parties. On Monday, Republicans abandoned lawsuits challenging election results in four battleground states – Michigan, Georgia, Pennsylvania and Wisconsin – where Mr Biden was projected as the winner.
In all four filings, ditched within one hour of each other, no reasons were provided for halting the legal action. Each case had been filed by voters – not by the Trump campaign or by Republican officials – though President Trump has continued to urge supporters to challenge election results.
Lawsuits were filed after the election to challenge vote counts that projected a loss for Mr Trump, but experts judged most to be on shaky legal ground.