As the government shutdown approached a month in duration, airports nationwide faced ongoing delays caused by persistent air traffic control staffing shortages.

Transportation Secretary Sean Duffy has repeatedly cautioned that travelers will encounter an increasing number of flight delays and cancellations. This warning comes as air traffic controllers continue their duties without remuneration since the shutdown commenced on October 1.

“The challenges are mounting day by day,” Duffy remarked to journalists on Thursday following a confidential meeting with Vice President JD Vance and key figures from the aviation sector, discussing the shutdown’s repercussions on U.S. travel.

The Federal Aviation Administration cited staffing-related delays on Friday at several airports, including those in Boston, New York City, Nashville, Houston, Dallas, and Newark, New Jersey. Delays at airports in Boston, Nashville, and New York City averaged two hours or more.

Aviation analytics company Cirium reported a noticeable “broader slowdown” in the U.S. aviation network on Thursday, marking the first occurrence since the shutdown began, which implies that disruptions due to staffing shortages might be expanding.

On Thursday, a significant number of major U.S. airports experienced lower-than-average punctuality, with fewer flights departing within 15 minutes of their scheduled times, as per Cirium’s data. However, the data does not differentiate between causes for delays, such as staffing issues or adverse weather conditions.

Staffing-related delays at Orlando’s airport on Thursday, for example, averaged nearly four and a half hours for some time, according to the FAA.

Most controllers are continuing to work mandatory overtime six days a week during the shutdown without pay, the National Air Traffic Controllers Association has said. That leaves little time for a side job to help cover bills, mortgage 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 paycheck 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.

Last weekend, a shortage of controllers 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.

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 shortage of about 3,000 air traffic controllers.