Six more US states have confirmed infections of the Omicron variant of Covid-19 but the Delta strain likely remains a greater threat as winter sets in and Americans gather for the holidays, experts said.

New Jersey, Maryland, Missouri, Nebraska, Pennsylvania and Utah each reported their first cases of the Omicron variant. Missouri was awaiting CDC confirmation of a case involving a St Louis resident who had recently travelled within the United States.

A surge in infections could further strain US hospitals already grappling with high caseloads and fatigued staff, Dr Ashish Jha, dean of Brown University's School of Public Health, told MSNBC.