The Omicron variant of the coronavirus has spread to about one-third of US states, but the Delta version is still providing the majority of COVID-19 infections as cases rise nationwide.

Though the emergence of the new variant has caused alarm worldwide, Doctor Anthony Fauci, the top US infectious disease official, told CNN, "Thus far it does not look like there's a great degree of severity to it."

He added it was too early to draw definitive conclusions and that more study is needed.

Fauci, US President Joe Biden's chief medical adviser, said he also hoped the United States would lift its ban on travellers from southern African countries in a "reasonable period of time".

The South African government has complained it is being punished - instead of applauded - for discovering the new variant and quickly informing international health officials.

Fauci, in an interview on CNN's State of the Union, praised South Africa for its transparency and said the travel ban was imposed at a time "when we were really in the dark" and needed time to study the variant.

At least 16 US states have reported Omicron cases: California, Colorado, Connecticut, Hawaii, Louisiana, Maryland, Massachusetts, Minnesota, Missouri, Nebraska, New Jersey, New York, Pennsylvania, Utah, Washington and Wisconsin.

Many of the cases were among fully vaccinated individuals and showed mild symptoms. The booster-shot status of some patients was not reported.

Despite several dozen Omicron cases, the Delta variant still accounts for 99.9 per cent of new COVID-19 cases in the United States , CDC Director Dr Rochelle Walensky told ABC News in an interview.

Over the last seven days the US has averaged 119,000 new cases and 1300 deaths linked to the virus each day.

Meanwhile, the emergence of the new variant has sharply curtailed the number of energy executives and government ministers planning to attend the four-day World Petroleum Congress in Houston this week, which had already been rescheduled from 2020.

Travel restrictions and worries over Omicron led energy ministers from Saudi Arabia, Kazakhstan, Qatar, Argentina, Equatorial Guinea, Greece, Turkey and Romania to bow out, WPC officials said on Sunday.

As Omicron cases emerge, vaccine makers aim to quickly tweak their shots to target the variant and US regulators have vowed speedy reviews, but that could still take months.

Moderna has targeted US approval of an updated vaccine as soon as March, but company officials said it will still take time to increase output.

Moderna co-founder Noubar Afeyan told CNN it would take another seven to 10 days to gather key data, and "a good 60 to 100 days" to deploy an Omicron-specific shot, although other options like a higher dose of the current booster are being explored.

US government officials are also working with Pfizer and Johnson & Johnson on updated shots, while Pfizer and Merck & Co are pursuing COVID-19 pill treatments.