New York Governor Kathy Hochul issued a Covid-19 "disaster emergency" declaration on Friday, citing increasing rates of infections and hospitalizations.
The discovery of a new coronavirus variant named Omicron triggered global alarm on Friday as countries rushed to suspend travel from southern Africa.
President Joe Biden said on Friday that the United States will bar entry to most travellers from eight southern African countries starting on Monday, after a potentially more-contagious new coronavirus variant was identified in South Africa.
The new variant, dubbed Omicron, poses a new challenge for Biden, who has had a mixed success getting Americans vaccinated after a politically motivated pushback by 10 states. Biden also faces criticism from international health experts and foreign leaders for failing to send vaccines to poorer countries.
The travel restrictions do not ban flights or apply to US citizens and lawful US permanent residents.
Countries around the world rushed to suspend travel from southern Africa after the World Health Organization said Omicron was "of concern." Many of those bans kick in immediately, unlike those issued by Biden.
The restrictions apply to South Africa, Botswana, Zimbabwe, Namibia, Lesotho, Eswatini, Mozambique and Malawi. Most non-US citizens who have been in those countries within the prior 14 days will not be allowed into the United States.
Biden made the announcement while spending the Thanksgiving holiday weekend on the Massachusetts island of Nantucket.
However, epidemiologists warned travel curbs may be too late to stop Omicron from circulating globally. The new mutations were first discovered in South Africa and have since been detected in Belgium, Botswana, Israel and Hong Kong.
The variant has a spike protein that is dramatically different than the one in the original coronavirus that vaccines are based on, the UK Health Security Agency said, raising fears about how current vaccines will fare.
Scientists issued similar warnings.
But it could take weeks for scientists to fully understand the variant's mutations and whether existing vaccines and treatments are effective against it.
Meanwhile, the World Health Organisation (WHO) said Omicron may spread more quickly than other forms, and preliminary evidence suggested there is an increased risk of reinfection.
Omicron is the fifth variant of concern designated by the WHO.