The United States has identified its first known case of Omicron, discovered in a fully vaccinated patient in California who travelled to South Africa, as scientists continue to study the risks the new COVID variant could pose.

Public health officials said the infected person, who had mild symptoms, returned to the United States from South Africa on November 22 and tested positive seven days later.

That patient was fully vaccinated but did not have a booster shot, according to Dr Anthony Fauci, the top US infectious disease official, who briefed reporters at the White House.

The person is in self-quarantine and all of the patient's close contacts have tested negative so far, he said.

Key questions remain about the new variant, which has rattled markets amid signs it may spread quickly and evade some of the defences provided by vaccines. It has been found in two dozen countries , including Spain, Canada, Britain, Austria and Portugal.

Fauci said it could take two weeks or more to gain insight into how easily the variant spread from person to person, how severe was the disease it caused and whether it could bypass the protections provided by vaccines currently available.

"We don't have enough information right now," said Fauci, who serves as an adviser to President Joe Biden, adding that the variant's molecular profile "suggests that it might be more transmissible, and that it might elude some of the protection of vaccines, but we don't know that now... We have to be prepared that there's going to be a diminution in protection."

For days, US health officials have said the new variant -first detected in southern Africa and announced on November 25 - was likely already in the United States as dozens of other countries also detected its presence.

The United States has barred nearly all foreigners who have been in one of eight southern African countries. On Tuesday, the US Centers for Disease Control and Prevention directed airlines to disclose names and other information of passengers who have been to those countries.