California Governor Gavin Newsom said his state had found its first known case of the new variant of the coronavirus, after the first US case was announced in Colorado.
Mr Newsom discussed the case, which was discovered in Southern California, during an online conversation with the country's leading infectious diseases expert, Dr Anthony Fauci, on Wednesday.
“I don’t think Californians should think that this is odd. It’s to be expected,” Dr Fauci said.
Mr Newsom did not provide any other details about the person who was infected with the mutated version of the virus in the country's most populous state.
This browser does not support the video element.
The Colorado and California cases have sparked questions about how the variant circulating in the UK arrived in the US and whether it was too late to stop it.
Top experts say it is probably already spreading elsewhere in the country.
The confirmed case in Colorado is a National Guardsman in his 20s who had not been travelling, officials said.
He has mild symptoms and is isolating at his home near Denver, while another National Guard member has a suspected case of the virus.
They had been sent in the past week to a nursing home that was struggling with an outbreak.
California is facing a growing pandemic crisis, including a record number of deaths.
Hospital resources are being increasingly stretched by soaring infections that are expected to grow in coming weeks.
Southern California and the San Joaquin Valley do not have the intensive-care capacity to treat patients suffering from the coronavirus, and state health officials are concerned over gatherings on New Year’s Eve.
But hope is on the horizon as vaccines are distributed.
And the statewide transmission rate has fallen to the point where one infected person is infecting only one other person, a development that Mr Newsom called encouraging.
But he warned Californians that rates in the central and southern parts of the state remained much higher and the trend could reverse with holiday gatherings.