The U.S. aviation system is expected to "remain challenged this summer and beyond" and the Federal Aviation Administration (FAA) needs more air traffic control staff, a senior United Airlines executive said Wednesday.

Travelers are already facing a tough summer with mounting flight cancellations amid rising demand as the industry tries try to rebuild staff levels after thousands of workers left during the COVID-19 pandemic.

"The reality is that there are just more flights scheduled industrywide than the (air traffic control) staffing system can handle," United's chief operations officer, Jon Roitman, said in a message to staff seen by Reuters.

"Until that is resolved, we expect the U.S. aviation system will remain challenged this summer and beyond."

U.S. Transportation Secretary Pete Buttigieg, who met June 16 with airline CEOs on summer travel issues, told CNBC on Tuesday that "air traffic control staffing issues do not explain the majority of delays and cancellations we've been seeing."

He cited as a key factor in flight issues the decision of airlines to push many pilots into early retirement despite $54 billion in COVID bailout funds.

U.S. airlines have cut about 15% of planned summer flights, an airline trade group said last month, while United has temporarily halted 12% of departures from its Newark hub alone because of congestion.

Over the four-day July 4th holiday weekend, 8.8 million passengers were screened by the U.S. Transportation Security Agency (TSA) checkpoints, topping 2020 and 2021 but around 12% lower than 2019 levels.

Roitman said United performed better than major rivals over the holiday weekend but said "we still have some work to do to meet standards of reliability and operational performance."

"Yes, weather and air traffic constraints impacted the entire industry, but we've built a culture of 'no excuses' and we clearly have room to improve," he added.

Last month, an airline industry trade group said the FAA must ensure adequate ATC staffing to avoid further summer travel disruptions, saying a key Florida ATC center had been "understaffed for 27 of the last 30 days, which is crippling to the entire east coast traffic flows."