California, Colorado and Minnesota have found cases of the variant among patients who were fully vaccinated and developed mild symptoms, while Hawaii reported a case with an unvaccinated person, who had moderate symptoms.
New York Governor Kathy Hochul on Thursday told a news conference one of the state's the cases involved a 67-year-old Long Island woman with mild symptoms of a headache and cough who had recently returned from South Africa.
The woman tested negative upon her return on November 25 but tested positive on Tuesday, and her results were sent to a lab for further examination.
She had "some vaccination history" but it wasn't yet clear if she had received one or two doses or a booster shot, Hochul said.
The other four people were New York City residents but further information was not yet available, the governor said.
The Minnesota patient is the first known US case of community transmission of the Omicron variant, found in a fully vaccinated man who had recently travelled to New York City and attended a conference.
Hawaii on Thursday also confirmed a case of community spread. The infected person, who had no history of travel, had previously had COVID-19, the Department of Health said.
To combat the spread of Omicron, President Joe Biden on Thursday announced new testing requirements for international travellers and promised in the coming weeks that Americans would have access to free, rapid at-home testing.
Colorado health officials on Thursday said a woman with the Omicron variant had recently returned from a trip to southern Africa.
California on Wednesday reported the first US case of the variant in a fully-vaccinated traveller who had been in South Africa. Los Angeles County on Thursday confirmed California's second case of the variant.
After determining the Minnesota case had a New York connection - the man attended an anime convention at New York City's Javits Center from November 19 to 21 - health officials immediately activated the "Test and Trace Corps" to contact conference attendees in an effort to contain the spread, Mayor Bill de Blasio said.
Dr. Leana Wen, a public health professor at George Washington University and former Baltimore health commissioner, said it was only a matter of time before more Omicron cases are detected in the United States.
The Minnesota case "means that there is spread in the United States. There will be many more cases to come," Wen said on Twitter.
Scientists are investigating Omicron, which has been labelled as a variant of concern by the World Health Organisation, to see if it is more transmissible than the Delta variant that is now prevalent and if it causes more severe disease. They are also studying how well current vaccines work against it.
About 60 per cent of the total US population, or 196 million people, have been fully vaccinated , one of the lowest rates among wealthy nations.Â
More than 786,000 people have died from COVID-19 in the United States, including 37,000 in November alone.
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