Washington, Jan 3 : More than 4,000 flights were cancelled globally in the past 24 hours, more than half of which were in the US, as new Covid-19 cases 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 them being within, into, or out of the US, reports Xinhua news agency. Another 4,519 US flights were delayed on Sunday. US news portal Axios said that some 4,700 domestic flights have been cancelled this weekend, with poor weather conditions contributing to delays already exacerbated by the Omicron variant in the US, where the Covid-19 record was shattered again on December 30 with over 580,000 daily cases reported nationwide. "The airport most affected by cancellations is Chicago O'Hare, where 273 flights -- 26 per cent 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 per cent 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. /IANS