Airlines cancelled several hundred US flights Saturday at the start of a long and almost certainly messy holiday travel weekend as carriers struggled to staff their planes.

As of mid-day, with Americans gearing up for July 4 Independence Day celebrations, some 600 flights within, into or out of the United States had been cancelled and more than 2,500 were delayed, according to flight tracking service flightaware.com.

The numbers on Friday were awful as well, with 587 US flights scrapped among a global total of 3,060 cancellations, the site said.

For days, amid a surge in travel as summer rolls in and people sick of cooped up pandemic life look to go places again, horror stories have abounded as travelers got stranded at airports, enduring odysseys to get where they are going.

The airline industry was devastated in the early stages of the pandemic as people stayed close to home.

And although federal Covid-19 relief spared airlines from laying off staff, tens of thousands of workers left the industry after carriers urged early retirements.

Today's industry has about 15 percent less staff compared with the pre-pandemic period to handle about 90 percent of pre-2020 passenger volume, analysts at Third Bridge consultancy estimated.

The travel chaos has drawn scrutiny from Transportation Secretary Pete Buttigieg and others in Washington.

On Saturday Buttigieg put out a series of tweets that were essentially consumer tips on what to do if one's flight is cancelled, like whether to accept travel points or miles as compensation or demand a cash refund.

"You can often negotiate on this. That's between you and the airline," Buttigieg wrote.

Delta pilots walked informational picket lines at several airports Thursday to demand a new contract and complain of overwork, among other gripes.

"Quite frankly, it's irresponsible scheduling, over scheduling. Coming out of the pandemic, we're scheduling more flights than we have people to fly them," Delta pilots association union leader Jason Ambrosi told CNN on Saturday.

"The pilots are getting fatigued, quite honestly," Ambrosi said. They do not want to strand travelers or crew members, he said, "but it's a safety issue."

Pilots are the most acute issue in a broad airline industry labor crunch, said Third Bridge analyst Peter McNally.

"There's no short-term fix," McNally told AFP. "The issue becomes most pronounced during these seasonal peaks."

Airlines say they're working to address the situation, recruiting pilots and other staff and trimming summer seat capacity by 15 percent.

While acknowledging the pilot supply problem, airline industry officials point to other exacerbating factors, including turbulent weather, increased staff absences due to Covid and insufficient personnel at flight traffic control at some sites.