More than 4,000 flights were cancelled globally on Sunday, more than half of which were in the United States, as new coronavirus infections driven by the Omicron variant continue to cause staff shortages at airlines and airports amid a busy holiday travel season.

Worldwide, 4,020 flights had been cancelled by Sunday afternoon, according to flight tracker FlightAware, with 2,393 of the cancelled journeys being within, into, or out of the United States.

Another 4,519 U.S. flights were delayed on Sunday.

U.S. news portal Axios said that some 4,700 domestic flights have been canceled this weekend, with poor weather conditions contributing to delays already exacerbated by the Omicron variant in the United States, where the COVID-19 record was shattered again on Thursday with over 580,000 daily cases reported nationwide.

"The airport most affected by cancellations is Chicago O'Hare, where 273 flights -- 26 percent of those planned -- were shuttered on Sunday, as the surrounding Cook County grapples with a record number of coronavirus infections and a snowstorm," reported Forbes on Sunday.

Regional carrier SkyWest Airlines cancelled 510 flights Sunday, or 21 percent of its scheduled trips, while airlines like Southwest, JetBlue and Delta also reported more than 100 cancellations each.

"Snowstorms, freezing temperatures and severe weather across the country have also compounded airlines' woes by preventing flights from operating as scheduled," said the business magazine.

Mass flight cancellations were first reported in the days before Christmas as both air travel and COVID-19 infections gained momentum.

Last week, Anthony Fauci, chief medical advisor to the White House, told MSNBC that the government should consider requiring coronavirus vaccines for domestic flights.

The surge of coronavirus cases reported in the United States has been driven by the Omicron variant, which early studies suggest is more transmissible with milder symptoms and makes up the majority of new infections in the country.