WASHINGTON: Thunderstorms and heavy volume are causing extensive delays in flight schedules for Boston, Washington, New York and Charlotte, North Carolina, prompting some lengthy ground stops or delays for inbound flights.

Aviation website FlightAware said there were more than 6,900 delayed U.S. flights and more than 1,500 flight cancellations.

U.S. Transportation Secretary Pete Buttigieg on Thursday urged airline executives to ensure summer flight schedules are followed after a recent spate of cancellations, a source told Reuters.

Buttigieg called the virtual meeting with U.S. airline CEOS after more than 2,700 flights were cancelled over the Memorial Day holiday, which prompted two U.S. senators to ask whether airlines were being held accountable for the disruptions.

He “pushed airlines to scrutinize whether they can reliably operate the schedules they have published and future schedules under consideration,” the source said.

Travelers are bracing for a difficult summer as airlines expect record demand and rebuild staff levels after thousands of workers left the industry during the COVID-19 pandemic.

Most planes are operating near full capacity and airlines often have less margin to address flight disruptions.

Buttigieg asked the chief executives to detail steps to address issues going into July 4th holiday and summer season and urged airlines to improve customer service to quickly accommodate customers after canceled flights, the source added.

Industry trade group Airlines for America (A4A), which confirmed the virtual meeting with Buttigieg, told Congress last week that the Federal Aviation Administration (FAA) must ensure the air traffic control system is capable of meeting demand.

Last month, Democratic Senators Richard Blumenthal and Edward Markey asked A4A, which represents American Airlines , Delta Air Lines, United Airlines and others, for answers on Memorial Day cancellations.

The FAA in May said it would boost authorized air traffic control staff at its Jacksonville, Florida, center after bad weather and space launches snarled flights.

The senators separately asked Buttigieg to detail steps his office was “taking to hold airlines accountable for serious disruptions and to ensure consumers are wholly and justly compensated.