Police officers stand guard as travellers arrive for hotel quarantine in Melbourne. AFP

Six more US states confirmed infections of the Omicron variant of COVID-19 on Friday 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 on Friday. Missouri was awaiting CDC confirmation of a case involving a St. Louis resident who had recently travelled within the United States.

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Canada has discovered a total of 15 cases of the new Omicron variant of COVID-19 and severe illness trends across the country could start to rise again, public health officials said on Friday.

The federal government said it backed a recommendation of a national advisory board on immunisations that all adults above 50 should receive a booster shot six months after completion of a vaccine series.

Pennsylvania and Utah each reported their first cases of the Omicron variant on Friday.

Ottawa announced last week it will require people arriving by air from all nations except the United States to take a COVID-19 test, and it expanded a ban on travellers from southern Africa to cover 10 nations.

Scientists are still investigating the impact of the highly contagious Omicron variant, which was first detected in South Africa. Early evidence has suggested it may cause milder illness than its predecessors, including Delta.

The outbreak of Omicron has made worldwide headlines and prompted political leaders to impose new COVID-19 restrictions. But the predominant U.S. strain remains Delta, Rochelle Walensky, director of the US Centers for Disease Control and Prevention (CDC), told a briefing at the White House.

The Utah case was discovered through ongoing genetic sequencing of positive COVID-19 samples at the state laboratory, the state's health department said on Twitter.

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The Omicron coronavirus variant spread in Australia on Saturday, testing plans to reopen the economy as a cluster in Sydney grew to 13 cases and an infection was suspected in the state of Queensland.

Federal authorities are sticking with a plan to reopen the economy on the hope that the new variant proves to be milder than previous strains, but some state and territory governments have moved to tighten their domestic border controls.

Australia reported its first community transmission of Omicron on Friday at a school in Sydney. Authorities are investigating the source.

Further Omicron cases were expected over the weekend when more tests results come, said Kerry Chant, chief health officer of New South Wales, of which Sydney is the capital.

Queensland authorities said the state suspected its first Omicron case in a person who travelled from South Africa and that genome sequencing was ongoing.