Airlines struggling to staff their planes cancelled hundreds of US flights Saturday at the start of a long and almost certainly messy holiday travel weekend.

As of mid-afternoon, with Americans gearing up for July 4 Independence Day celebrations, more than 600 flights within, into, or out of the United States had been cancelled, and more than 3,300 were delayed, according to flight tracking service flightaware.com.

The numbers on Friday were grim as well, with 587 US flights scrapped among a global total of 3,061 cancellations, the site said. Sunday was also looking problematic, with more than 100 flights already cancelled.

The airport chaos is prompting a record level of road travel by Americans seeking to dodge flight trouble, a travel industry group said.

For days, amid a surge in travel as summer rolls in, horror stories have abounded as travelers were stranded at airports, enduring odysseys to reach their destinations.

The airline industry was devastated in the early stages of the Covid-19 pandemic as people stayed close to home, but air travel has rebounded as health measures were eased.

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

Today's industry has about 15 percent less staff compared with the pre-pandemic period to handle around 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 tweeted a series of tips on what to do if one's flight is cancelled, such as 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.

Travelers pick up their baggage while arriving at Ronald Reagan Washington National Airport in Arlington, Virginia, on July 2, 2022
Travelers pick up their baggage while arriving at Ronald Reagan Washington National Airport in Arlington, Virginia, on July 2, 2022 AFP / Stefani Reynolds

The travel season is at full speed, with 2,490,490 people screened at airport checkpoints nationwide on Friday, the most since February 2020 right before the Covid shutdown in the US, the Transportation Security Administration said.

"We are back to pre-pandemic checkpoint volume," the TSA tweeted.

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

"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 added, "but it's a safety issue."

Lack of pilots is the most acute problem 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 shortage, airline industry officials point to other exacerbating factors, including turbulent weather, increased staff absences due to Covid and insufficient flight traffic control personnel at some sites.

For the long Independence Day weekend, a record 42 million Americans will also travel by road at least 50 miles (80 kilometers) from home, despite soaring gasoline prices, the American Automobile Association said.

The travel hassles affecting the airline industry may be fueling the heavy road traffic, it noted.

"Traveling by car does provide a level of comfort and flexibility that people may be looking for given the recent challenges with flying," said AAA Travel senior vice president Paula Twidale.