- Joe Biden has warned "people may die" if his incoming presidential administration continues to be impeded by Donald Trump.
Speaking in Delaware, the president-elect said co-ordination was needed to tackle the coronavirus outbreak.
He again called President Trump's refusal to acknowledge he lost the vote, despite calls to do so from both sides, "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, a Republican, tweeted on Monday morning: "I won the Election!"
The Trump campaign launched a flurry of legal challenges in the wake of the 3 November vote to contest ballot counts.
The General Services Administration (GSA), the government agency tasked with beginning the transition process for a new president, has yet to recognise 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 Democratic president-elect have said that Mr Trump's refusal to engage in a 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. Calling nationwide vaccine distribution a "huge, huge undertaking", Mr Biden said that if his team had to wait until 20 January - his presidential inauguration - until they could begin work on the distribution programme, they would be behind by "over a month, month and a half".
Asked if he would encourage state leaders to reinstate stay-at-home orders, the president-elect sidestepped, and instead called on officials to encourage mask-wearing.
Is pressure growing for Trump to concede?
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 given 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 by the Trump campaign after the election to challenge vote counts that projected a loss for the president, but experts judged most to be on shaky legal ground.
One White House, different views
National Security Adviser Robert O'Brien spoke of a "professional transition" to the next administration on Monday. His tone was different from that of Secretary of State Mike Pompeo, who spoke of a "second Trump administration" last week, and others who act as though the president will remain.
When I asked Judd Deere, a spokesman here at the White House, about Mr O'Brien's remarks and his own role in the transition, Mr Deere shot the idea down: "There's not a transition at this point." When I asked about advice he'd give to those who will come after him, he got a bit testy: "I don't speak in hypotheticals."
His remarks reinforce those of the secretary of state, but clash with those of the national security adviser, an unsurprising development, given that their boss is himself a bundle of contradictions. He has tweeted about a Biden victory one moment and then reversed himself, and his cabinet secretary, adviser and others reflect his zig-zaggy approach.
Former US First Lady Michelle Obama also issued a pointed rebuke to Mr Trump on Instagram.
She described the difficulty of welcoming Mr Trump to the White House four years ago after he "spread racist lies about my husband that had put my family in danger" - referring to bogus claims the current president amplified about Mr Obama's birthplace.
However, "our love of country requires us to respect the results of an election even when we don't like them or wish it had gone differently - the presidency doesn't belong to any one individual or any one party", Mrs Obama wrote.
"To pretend that it does, to play along with these groundless conspiracy theories - whether for personal or political gain - is to put our country's health and security in danger. This isn't a game," she said.
Mrs Obama called on Americans, "especially our nation's leaders, regardless of party", to "honour the electoral process and do your part to encourage a smooth transition of power".
"The American people had spoken. And one of the great responsibilities of the presidency is to listen when they do," she wrote.
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from Via PakapNews