Travelers should prepare for potential flight disruptions this holiday weekend as the government shutdown continues, a group representing the largest U.S. airlines said Friday.

Air traffic controller shortages this week delayed flights at some U.S. airports, including in Burbank, California, and Nashville, Tennessee.

“It is safe to fly, but ATC staffing shortages strain the system and cause flights to be spaced out, slowing down everything. In some cases, flights may be delayed or even cancelled,” said Airlines for America, whose members include Delta Air Lines, United Airlines, American Airlines and others.

Bad weather could also snarl travel over the long weekend. The National Weather Service on Friday issued a flood watch for New York, Long Island, and parts of Connecticut and New Jersey from an expected storm.

During a federal government shutdown, “essential” workers such as air traffic controllers and TSA agents worth without pay, while many other employees are placed on furlough.

A more than monthlong shutdown starting in late 2018 ended hours after a shortage of air traffic controllers snarled air travel in the New York area.

Absences of air traffic controllers rose this week as the current shutdown stretched into its second week, Bryan Bedford, head of the Federal Aviation Administration, reiterated to staff on Friday.

“Air traffic controllers are still required to report to work and carry out their critical duties. The safety and efficiency of our airspace depend on them,” he said in a note to employees, which was seen by CNBC.

Delta CEO Ed Bastian earlier this week told CNBC that the carrier has so far not seen “any impacts at all” to its operation because of the shutdown. However, he said that could change if it continued for another roughly 10 days.

On Friday, close to 3,700 U.S. flights were delayed, according to flight-tracking site FlightAware, below the average daily rate of about 4,100 for U.S. airlines so far this year.