WASHINGTON, Dec 22 (AFP) - Over 4,400 flights have been cancelled over a two-day period as a powerful winter storm hits the United States, coinciding with the start of a holiday season that some predict could be the busiest ever.

More than 2,350 US flights had been cancelled on Thursday (Dec 22) and another 2,120 flights for Friday were scrapped, according to flight tracking website FlightAware, while passenger railroad Amtrak cancelled dozens of trains through Christmas, disrupting holiday travel for tens of thousands.

Another 8,450 flights were delayed on Thursday - including more than one-third of those operated by American Airlines, United Airlines, and Southwest Airlines.

Southwest cancelled 865 flights on Thursday, about one-fifth of all its scheduled flights, and had already scrapped another 550 for Friday.

The Federal Aviation Administration said on Thursday that the winter storm was bringing blizzard conditions to the Midwest, with major travel disruptions expected in Chicago, Detroit and Minneapolis-St Paul.

Delta Air Lines, which had cancelled 140 flights on Thursday out of 4,400 and 90 on Friday per FlightAware, warned "additional cancellations will be necessary Friday as the storm continues to impact operations in Detroit and the Northeast".

As of 7.30pm ET on Thursday (8.30am, Friday, Singapore time), 25 per cent of departing flights at Chicago's O'Hare International Airport and 37 per cent of flights at Chicago Midway were cancelled, while 27 per cent of departing Denver flights had been cancelled.

Amtrak said it was cancelling several dozen scheduled train trips in the Midwest through Christmas because of the weather conditions, including trains in Michigan, Illinois, and Missouri and trains between New York and Chicago.

Brandon Mattis, 24, was at New York's La Guardia Airport seeking to get to Atlanta, Georgia, to join the rest of his family for Christmas celebrations. His flight had been cancelled, he said.

"We're trying to search on our phones. Figure out other routes. Maybe even taking a bus from here to Atlanta, which it'll take us about 21 hours. So, that's really inconvenient. But anything we can do just to get there (is) what we're going to do."

In the seven days ending on Wednesday, the Transportation Security Administration said it screened nearly 16.2 million passengers, slightly below the 16.5 million screened in the same period in 2019, pre-COVID pandemic.

Last year's holiday period was marred by an outbreak of COVID-19 among staff that forced airlines to cancel thousands of flights.

US airlines said earlier this week they were waiving change fees and fare differences for passengers in a range of affected areas.