South Carolina health officials confirmed they had found the US's first two people to be infected with the variant of Covid-19 first identified in South Africa.
“The arrival of the variant in our state is an important reminder to all South Carolinians that the fight against this deadly virus is far from over,” said Dr Brannon Traxler, interim director at the South Carolina Department of Health and Environmental Control.
There are fears that the variant may be more transmissible and deadlier.
The people in South Carolina infected with the new B.1.351 variant did not travel recently and the two cases were not related, suggesting community spread.
It is expected that this variant, with others, has been spreading undetected in the US, because the country lacks surveillance programmes.
The cases in South Carolina also confirm that all three "variants of concern", so labelled by the World Health Organisation, are now spreading in the US.
Cases of the UK and Brazil variants have been found in other US states.
This week, the state of Minnesota said a case of the Brazilian variant had been detected in a patient who had returned from travelling in the country.
The UK variant was first found in Colorado in December last year.
The Centres for Disease Control and Prevention is aware of the South Carolina cases and advised people to continue taking precautions wherever possible.
“At this time, we have no evidence that infections by this variant cause more severe disease,” the agency said.
“Like the UK and Brazilian variants, preliminary data suggests this variant may spread more easily and quickly than other variants.”
The state of California is also looking into confirming the existence of another variant, L452R.
It is normal for viruses to mutate and create new strains but these "variants of concern" alarmed scientists because they could be more easily transmitted.
Research is being done to see if they are deadlier than the original strain or if they pose challenges to Covid-19 vaccines.
“While more Covid-19 vaccines are on the way, supplies are still limited," Dr Traxler said.
"Every one of us must recommit to the fight by recognising that we are all on the front lines now. We are all in this together.”
At least 25 million people have been infected with the virus in the US, data compiled by Johns Hopkins University shows.
More than 430,000 people in the US died from Covid-19 and that number is expected to pass half a million in mid-February.