More than 1,500 flights across the globe were scrapped on Sunday, bringing more misery for travellers planning to fly over the festive period.
As many as 5,900 flights have now been cancelled on Christmas Eve, Christmas Day and the day after, according to FlightAware data tracking website.
Chinese and US airlines appear to be the hardest hit. Further disruption is expected on Monday.
Companies have blamed staff shortages on the fast-spreading Omicron variant.
They say flight cancellations are the result of airline crews testing positive, or being forced to self-isolate.
Despite early findings that Omicron is milder than other variants, scientists are concerned by the sheer number of infections being recorded.
More than 450 flights to or from US airports have been cancelled on Sunday, according to FlightAware.
The worst-hit US companies are Delta, United and JetBlue.
United warned earlier that a spike in Omicron cases had "had a direct impact on our flight crews and the people who run our operation", adding that it was contacting impacted passengers in advance of them coming to the airport.
Omicron now is the dominant strain in the US.
However, the most affected single airline appears to be China Eastern with more than 350 scrapped flights on Sunday.
The airport in the northern Chinese city of Xi'an has reported nearly 100 flight cancellations - the highest number so far.
More than 13 million people in Xi'an have been recently ordered to stay at home as authorities attempt to tackle a Covid outbreak there.
Heathrow airport in London has seen 56 flights cancelled on Sunday so far.
Overall, global airlines have cancelled about 5,700 flights since Friday.
Nearly 5.4 million people have died with coronavirus worldwide, according to America's Johns Hopkins University. There have been more than 279 million confirmed cases.
BDST: 1246 HRS, DEC 26, 2021