Travel delays were adding up at airports across the U.S. on Friday as the government shutdown drags on, putting even more pressure on air traffic controllers who have been working without pay for a month.

U.S. Transportation Secretary Sean Duffy has been warning that travelers will start to see more flight disruptions the longer controllers go without a paycheck.

“Every day there’s going to be more challenges,” Duffy told reporters Thursday outside the White House after a closed-door meeting with Vice President JD Vance and aviation industry leaders to talk about the shutdown’s impact on U.S. travel.

The Federal Aviation Administration on Friday reported staffing shortages that were causing flight delays at a number of airports, including in Boston, Phoenix, San Francisco, Nashville, Houston, Dallas and the Washington, D.C. area. Airports serving the New York City area — John F. Kennedy International Airport, LaGuardia Airport and Newark Liberty International Airport — were also experiencing delays averaging around two hours, according to the FAA.

“Currently nearly 50 percent of major air traffic control facilities are experiencing staffing shortages, and nearly 90 percent of air traffic controllers are out at New York–area facilities,” the FAA said in a statement posted on X on Friday evening.

Staffing shortages can occur both in regional control centers that manage multiple airports and in individual airport towers, but they don’t always lead to flight disruptions. According to aviation analytics firm Cirium, flight data showed strong on-time performance at most major U.S. airports for the month of October despite isolated staffing problems that surfaced throughout the month.

But Cirium said the data also showed a “broader slowdown” Thursday across the nation’s aviation system for the first time since the shutdown began on Oct. 1, suggesting staffing-related disruptions may be spreading.

According to Cirium, many major U.S. airports on Thursday saw below-average on-time performance, with fewer flights departing within 15 minutes of their scheduled departure times. Staffing-related delays at Orlando’s airport on Thursday, for example, averaged nearly four and a half hours for some time. The data does not distinguish between the different causes of delays, such as staffing shortages or bad weather.

Last weekend, a shortage of controllers also led to the FAA issuing a brief ground stop at Los Angeles International Airport, one of the busiest in the world. Flights were held at their originating airports for about two hours Sunday until the FAA lifted the ground stop.

Most controllers are continuing to work mandatory overtime six days a week during the shutdown, the National Air Traffic Controllers Association said. That leaves little time for a side job to help cover bills, mortgage payments and other expenses unless controllers call out.

Duffy said controllers are also struggling to get to work because they can’t afford to fill up their cars with gas. Controllers missed their first full paychecks on Tuesday.

“For this nation’s air traffic controllers, missing just one paycheck can be a significant hardship, as it is for all working Americans. Asking them to go without a full month’s pay or more is simply not sustainable,” Nick Daniels, president of NATCA, said Friday in a statement.

Some U.S. airports have stepped in to provide food donations and other support for federal aviation employees working without pay, including controllers and Transportation Security Administration agents.

Before the shutdown, the FAA was already dealing with a long-standing shortage of about 3,000 air traffic controllers.