Washington, Dec 26 : The holiday season travel stress has only escalated in the US amid the surge of the Omicron variant of Covid-19, with several airlines reporting staff shortages from the spread of the coronavirus, leaving passengers with delayed or cancelled flights. According to FlightAware, a flight-tracking site, 880 flights within, in or out of the US were cancelled on December 25, which were among the approximately 2,521 flights that have been cancelled so far this day worldwide, Xinhua news agency quoted National Public Radio (NPR) as saying in a report on Saturday. "US-based airlines such as Delta, United and JetBlue are also seeing high cancellation numbers," the report said. Delta alone has cancelled 290, or 14 per cent, of its Christmas Day flights, and American Airlines (AAL) cancelled 88 flights on Saturday. Their operations have been "running smoothly" but that Covid-related sick calls led them to pre-cancel some of their scheduled flights for the day, and affected customers were notified the previous day, AAL was quoted as saying in a statement. "While airline travel was starting to pick back up before the Omicron variant emerged, the massive spike in positive Covid-19 cases seen in recent weeks has become a cause for concern to airlines in keeping airline staff and passengers safe," the NPR report said. Lawmakers are calling on the Centers for Disease Control and Prevention (CDC) and the Federal Aviation Administration (FAA) to make a major change and require proof of vaccination or a negative Covid-19 test in order to travel on all domestic flights, it added. "Ensuring the health and safety of air travellers and their destination communities is critical to mitigating the ongoing Covid-19 surge, especially as the virus continues to evolve," the lawmakers wrote in a letter to the CDC and FAA on Monday. Vaccination protocols for international travellers coming into the US are already in place, according to the CDC. /IANS