Travel chaos is dogging US airports on the eve of Independence Day, with more than 100 million Americans under extreme weather warnings. More than 1,400 flights were delayed or canceled on Monday. It follows one of the busiest air travel weekends on record. Sunday saw the highest number of passengers ever screened at US airports. Severe 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 200 postponed flights, according to data from the FlightAware website. United experienced more than 5,000 delays and cancelations over the past week, far outpacing those of any other US airline. Its chief executive, Scott Kirby, was forced to apologize on Friday after he chartered a private jet from New York to Colorado to avoid being held up by the airline"s delays. In a letter to employees on Saturday, he said thunderstorms at United"s largest hub, in New Jersey, had created an "extended limited operating environment" and "one of the most operationally challenging weeks I"ve experienced in my entire career". The airline is compensating customers worst hit by the chaos with 30,000 frequent flier miles — although one Atlanta family stuck in three different airports over five days said it was not much of an apology. 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 180,000 residents in the US Midwest were still without power after storms over the weekend, including over 50,000 people in Missouri. — BBC