WASHINGTON (Reuters) -The Federal Aviation Administration said on Friday that air traffic control staffing issues delayed travel for a fifth straight day as major U.S. airlines urged air travelers to be patient during an ongoing government shutdown.

More than 6,400 flights were delayed on Friday - and 470 canceled - after 22,000 had been delayed since Monday, according to flight tracking website FlightAware. Air travel is expected to rise in the United States during the three-day Columbus Day holiday weekend.

The FAA said late on Friday it was experiencing staffing issues in Chicago, New York, Newark, San Diego, Dallas, Washington and Phoenix, among other airports. A temporary evacuation of the Atlanta air traffic control tower Friday due to a fire alarm and reports of a strong natural gas odor also snarled flights.

Airlines were urging passengers to be patient.

"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."

Air traffic control has become a flashpoint in the debate over the shutdown with both parties blaming the other. The Trump administration on Thursday began airing videos at security checkpoints blaming Democrats.

The head of the FAA said earlier 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.

The U.S. has faced an air traffic controller staffing shortage for more than a decade, and many had been working mandatory overtime and six-day weeks even before the shutdown. The FAA is about 3,500 air traffic controllers short of targeted staffing levels.

U.S. Transportation Secretary Sean Duffy warned on Thursday that air traffic controllers who repeatedly fail to show up for work during the government shutdown could be fired.