Travel chaos is plaguing US airports on the eve of Independence Day, with more than 100 million Americans under severe weather warnings.

More than 1,400 flights were delayed or canceled as of Monday morning.

It comes after one of the busiest air travel weekends on record. Sunday saw the highest number of passengers ever screened at US airports.

Violent storms threaten large swaths of the eastern United States, while the southern and western regions continue to be scorching hot.

United Airlines remained the airline hardest hit by the delay on Monday, accounting for more than 200 flights delayed, according to data from website FlightAware.

United experienced more than 5,000 delays and cancellations in the past week, outpacing any other US airline.

Its CEO, Scott Kirby, was forced to apologize on Friday after he chartered a private jet from New York to Colorado to avoid being hit by airline delays.

In a letter to staff on Saturday, he said thunderstorms at United’s largest hub, in New Jersey, had created a “prolonged limited operating environment” and “one of the most challenging weeks most active I’ve been through in my entire career.”

Transport Secretary Pete Buttigieg on Sunday said the reason US travel was “chaotic” was due to extreme weather, which he said had “put enormous pressure on the system” “.

He told CBS that the Federal Aviation Administration would hire 1,500 new air traffic controllers this year and another 1,800 next year.

Entering the long weekend of July 4, industry officials were concerned that the new 5G deployment around airports could interfere with aircraft technology.

But a Department of Transportation spokesman told CBS News there were no major flight disruptions related to the matter.

Thunderstorms – some accompanied by heavy hail – are forecast for Monday across the east from Mississippi to Massachusetts, as well as in states further north like Montana and Minnesota.

The heatwave across the southern US will remain in force, with record numbers expected to move up the west coast to California and Oregon.

More than 180,000 residents in the U.S. Midwest are still without power after the storms over the weekend, including more than 50,000 in Missouri.