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

More than 3,000 flights were delayed or cancelled within, into, or out of the US on Monday, according to FlightAware.

Friday saw the highest number of US airport passengers ever, Transportation Security Administration data showed.

Storms threaten a swathe of the eastern US, while the south and west continue to bake in a heatwave.

United Airlines remained the carrier worst affected by the delays on Monday, accounting for over 300 postponed flights, according to data from the FlightAware website.

The carrier experienced more than 5,000 delays and cancellations over the past week, far outpacing those of any other US airline.

In a letter to employees on Saturday, chief executive Scott Kirby said thunderstorms at United's largest hub, in Newark, New Jersey, had created an "extended limited operating environment".

The situation was "one of the most operationally challenging weeks I've experienced in my entire career," Mr Kirby said.

The airline said it was compensating customers worst hit by the chaos with 30,000 frequent flier miles - estimated to be worth $360 (£283) by Time Magazine.

But the offer did little to assist .

It came after Mr Kirby was forced to apologise on Friday after he chartered a private jet from New York state to Colorado amid the disruption.

Secretary of Transportation Pete Buttigieg said on Sunday the reason US travel has been "so chaotic" is because of severe weather, which he said had "put enormous pressure on the system".

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

Heading into the Fourth of July long weekend, industry officials had been concerned that a new 5G rollout around airports might interfere with aircraft technology.

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

Thunderstorms - some with large hail - were forecast on Monday across the east from Mississippi to Massachusetts, as well as in states further north such as Montana and Minnesota.

The heatwave across the US south will remain in effect, with record-breaking numbers expected to shift up the west coast into California and Oregon.

More than 150,000 residents in the US Midwest were still without power after storms over the weekend, including over 40,000 people in Missouri.