Dr Anthony Fauci has apologised for appearing to criticise the UK's vaccine approval process.
The top US infectious disease expert had previously told CBS News the UK "rushed" approving the Pfizer coronavirus vaccine.
"I have a great deal of confidence in what the UK does both scientifically and from a regulator standpoint," Dr Fauci told the BBC on Thursday.
The UK on Wednesday became the first country in the world to approve the Pfizer vaccine for the coronavirus.