Washington:US President-elect Joe Biden has criticised the Donald Trump-led administration's distribution of coronavirus vaccines, saying it was falling behind schedule.
Biden said the vaccine drive was the "greatest operational challenge we've ever faced as a nation", the BBC reported.
The Trump administration had aimed to vaccinate 20 million Americans by the end of December.
So far, only 2.1 million have received shots, according to the US Centers for Disease Control and Prevention (CDC).
"The Trump administration's plan to distribute vaccines is falling far behind," Biden said in a speech on Tuesday. "I'm going to move Heaven and Earth to get us going in the right direction."
Responding to Biden in a tweet, President Donald Trump said it was "up to the states to distribute the vaccines" once they had been delivered by the federal government.
"We have not only developed the vaccines, including putting up money to move the process along quickly, but gotten them to the states," he wrote.
Biden has pledged to vaccinate 100 million Americans during the first 100 days of his presidency when he takes office on 20 January.