US President Joe Biden has called the Omicron Covid variant a "cause for concern, not a cause for panic" one day after it was detected in North America.
He said he saw no need for a lockdown "for now", adding, "if people are vaccinated and wear their masks".
Cases have been found in Canada, and the US has imposed travel bans on eight African countries.
Despite vaccines and Mr Biden's vow to "shut down" the virus, more Americans died this year from Covid than last.
At the White House on Monday, the president called it "almost inevitable" that the Omicron strain, first reported by South Africa, will be found in the US "at some point".
He added that vaccine companies are creating "contingency plans" for new vaccines "if needed".
Late last week, the US announced a ban on flights from South Africa, Botswana, Zimbabwe, Namibia, Lesotho, Eswatini, Mozambique and Malawi. Canada, the UK and the EU and other countries have also restricted travel from southern Africa.
Canada, the US neighbour to the north, said on Sunday that the Omicron strain had been discovered in two patients who had recently travelled to Nigeria. A third case was announced on Monday.
Mr Biden said that the ban had bought some time for the US to study the new strain. While the World Health Organization has deemed Omicron a "variant of concern", it is still not clear whether it is associated with more transmission or more risk of evading vaccines.
Critics noted that Mr Biden accused former President Donald Trump of "hysterical xenophobia" after he limited travel from China in January 2020 as the coronavirus spread in the US.
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