Airlines canceled over 700 flights in the United States on Monday, as adverse weather and a shortage of staff hurt their ability to keep up with a surge in summer travel demand.

Total flight cancellations within, into, or out of the United States as of 1.07 p.m. ET were 747, as per flight-tracking website Flightaware.com. Nearly 860 flights were canceled on Sunday.

Delta Air Lines had over 200 cancellations, Republic Airways Inc and United Airlines Holdings Inc had 196 and 122 flight cancellations, respectively, while American Airlines Group Inc canceled 62 flights as of Monday afternoon.

American Airlines said the cancellations were largely due to weather and air traffic control initiatives designed to regulate traffic.

Delta, United and Republic did not respond to Reuters requests for comment.

In Europe, recent airport snarls have been blamed on a shortage of employees, as many workers, who were laid off during the pandemic, desert airport work for flexible working practices and other occupations.

Even the U.S. regulator Federal Aviation Administration (FAA) faces staff shortages.

The FAA last week granted United approval to temporarily cut Newark flights after the Chicago-based carrier petitioned for a waiver, citing airport construction and air traffic control staffing.

Airlines for America, a trade group, said on Friday the FAA must ensure adequate air traffic control staffing to avoid further summer travel disruptions.

Shares of Delta, United and American Airlines were down 2% to 3% in afternoon trade.