United States airlines cancelled hundreds of flights on Christmas Eve amid a surge in Covid-19 infections but millions of Americans carried on with their travel plans.

United Airlines and Delta Air Lines both cancelled dozens of flights because of staff shortages amid the surge in infections of the Omicron variant of Covid-19.

Delta cancelled 138 flights and United Airlines cancelled 170 flights, while more than 2,000 flights both in and out of the US were scrapped.

Both airlines said they would be working to contact passengers so they would not be stranded at airports.

"The nationwide spike in Omicron cases this week has had a direct impact on our flight crews and the people who run our operation. As a result, we've unfortunately had to cancel some flights and are notifying impacted customers in advance of them coming to the airport," United said.

Delta said it has "exhausted all options and resources" which included rerouting and substitutions of aircraft and crews to cover scheduled flying, before canceling flights.

But as the majority of flights went ahead, American passengers put aside their concerns over infection and continued with their plans.

Passengers reported "just wanting to get out of the house" and not wanting to miss out on arrangements as their reason for travelling.

Covid-19 infections have surged in the US in recent days due to Omicron, and the new variant now accounts for over half of US cases and as many as 90% of overall cases in some areas.

US health officials have said people who are fully vaccinated should feel comfortable proceeding with holiday travel and family gatherings, but warned the unvaccinated that they are at higher risk of getting seriously ill or dying from the virus.

Delta cited potential inclement weather and the impact of the Omicron variant for the cancellations.

On Tuesday, Delta Chief Executive Ed Bastian asked the head of the US Centers for Disease Control and Prevention (CDC) to shrink quarantine guidelines for fully vaccinated individuals who experience breakthrough COVID-19 infections, citing the impact on the carrier's workforce.

Bastian asked that the isolation period be cut to five days from the current ten.

That request was echoed both by Airlines for America, a trade group representing major cargo and passenger carriers, which wrote to the CDC on Thursday, and by JetBlue on Wednesday.

The CDC released updated quarantine guidance for healthcare workers on Thursday, cutting the isolation time to seven days for workers who test positive for Covid-19 but are asymptomatic, providing they test negative.