Major airlines cancelled thousands of flights scheduled to come into and out of the East Coast of the United States as a potentially historic blizzard is expected to dump as much as three feet of snow and snarl transportation for tens of millions of people.

The National Weather Service issued a blizzard warning for New York City and surrounding areas beginning 1 p.m. ET on Monday, and warned of two days of winter storms across the East Coast, from Pennsylvania to Maine.

“This could be the biggest snowstorm in the history of this city,” New York Mayor Bill de Blasio said.

Flight-tracking website FlightAware.com noted in a social media post Monday morning that around 4,000 flights have been cancelled for Monday and Tuesday. The post also said that almost all New York City flights will be cancelled Tuesday.

American Airlines Group AAL canceled all late afternoon and evening flights scheduled for Monday from the New York area, Philadelphia and Boston, spokeswoman Andrea Huguely said.

The cancellations affect flights of American Airlines, US Airways and all regional partners, Huguely said in an email. The company plans to resume operations “as soon as it is safe to do so,” she said.

As of early Monday afternoon, the company had canceled 626 flights that were scheduled for Monday and 970 scheduled for Tuesday, she said.

Delta Air Lines DAL said on Sunday it planned to cancel 600 flights because of the blizzard warning, while United Airlines UAL said it will cancel all Tuesday flights at airports in New York, Boston and Philadelphia. Beginning on Monday night, the carrier will limit operations at Newark, LaGuardia and John F. Kennedy airports in the New York area, a spokeswoman said.

Southwest Airlines LUV said Sunday evening it would cancel more than 130 of 3,410 flights scheduled for Monday due to the storm, an increase from its earlier plan to cancel about 20 flights.

Nearly all of the major U.S. carriers have waived the change fee for customers flying from affected cities during the storm, reported USA Today earlier Monday.

The Greater New York Taxi Association, an organization of taxi operators in New York City, said its members will offer free taxi rides to emergency workers unable to get to work and the elderly who might be stranded by the snow.