"This President will spare no expense to ensure that any new vaccine maintains our own FDA's gold standard for safety and efficacy, is thoroughly tested, and saves lives," he said.
The decision cuts the US off from securing doses from a group of possible vaccines.
"America is taking a huge gamble by taking a go-it-alone strategy," Lawrence Gostin, a professor of global health law at Georgetown University, told the Washington Post.
US Food and Drug Administration Commissioner Stephen Hahn told the Financial Times in an interview that the FDA was prepared to authorise a coronavirus vaccine before late-stage Phase Three clinical trials were complete, as long as officials are convinced that the benefits outweigh the risks.
Yesterday, WHO officials said moving too quickly to make a vaccine widely available could pose risks.
"If you move too quickly to vaccinate ... millions of people, you may miss certain adverse effects," said Mike Ryan, the head of WHO's emergencies programme.