WASHINGTON (Reuters) -A group representing major U.S. airlines on Friday urged air travelers to be patient as air traffic control staffing issues delayed travel for a fifth straight day.

"It is safe to fly, but ATC staffing shortages strain the system and cause flights to be spaced out, slowing down everything," said Airlines for America, the trade group representing American Airlines, United Airlines, Delta Air Lines, Southwest Airlines and other major carriers, warning of delays or cancellations.

"The bottom line is that anyone heading to the airport right now is encouraged to pack their patience."

The Federal Aviation Administration said staffing issues were resulting in delays at airports including Phoenix, Newark and New York LaGuardia and more are expected later in the day.

More than 2,500 flights have been delayed by 1 p.m. ET (1700 GMT) Friday, after 22,000 had been delayed since Monday. Air travel is expected to rise in the United States during the three-day Columbus Day holiday weekend.

Earlier, the head of the FAA said that the agency is facing continuing air traffic control staffing shortages as a U.S. government shutdown entered its 10th day but emphasized safety would not be compromised.

"Staffing shortages have increased across the country," FAA Administrator Bryan Bedford said in an email to employees. "Safety will never be compromised. When staffing constraints occur, we will reduce the flow of air traffic into affected airports and centers to maintain safe operations."

Some 13,000 air traffic controllers and about 50,000 Transportation Security Administration officers must still turn up for work during the government shutdown, but they are not being paid. Controllers are set to receive a partial paycheck on October 14 for work performed before the shutdown.

In 2019, during a 35-day shutdown, the number of absences by controllers and TSA officers rose as workers missed paychecks, extending checkpoint wait times at some airports. Authorities were forced to slow air traffic in New York, which put pressure on lawmakers to quickly end the standoff.