Staffing shortages led to more flight delays at airports across the U.S. on Tuesday as the federal government shutdown stretched into a seventh day, while union leaders for air traffic controllers and airport security screeners warned the situation was likely to get worse.

The Federal Aviation Administration reported staffing issues at airports in Nashville, Boston, Chicago and Philadelphia, and at its air traffic control centers in Atlanta and the Dallas-Fort Worth area. The agency temporarily slowed takeoffs of planes headed to the first three cities.

Major flight delays a day earlier also were tied to insufficient staffing during the shutdown, which began Oct. 1. The FAA reported delays on Monday at the airports in Burbank, California, Newark, New Jersey and Denver.

Travel industry analyst Henry Harteveldt said the risk of significant disruptions to the U.S. aviation system "is growing by the day" as federal workers whose jobs are deemed critical continue working without pay. The longer the shutdown drags on, the more likely it is to affect holiday travel plans in November, he said.

"I'm gravely concerned that if the government remains shut down then, that it could disrupt, and possibly ruin, millions of Americans' Thanksgiving holidays," Harteveldt said in a statement.

Transportation Secretary Sean Duffy said Monday that there has already been an uptick in air traffic controllers calling out sick in a few locations. When there aren't enough controllers, the FAA must reduce the number of takeoffs and landings to maintain safety, which in turn causes flight delays and possible cancellations.

That's what happened Monday afternoon, when the control tower at Southern California's Hollywood Burbank Airport shut down for several hours, leading to average delays of two-and-a-half hours.

When a pilot preparing for takeoff radioed the tower, according to communications recorded by LiveATC.net, he was told: "The tower is closed due to staffing."

Nick Daniels, president of the National Air Traffic Controllers Association, said the shutdown highlighted some of his union's members already face on a regular basis due to a national airspace system that is critically understaffed and relies on outdated equipment that tends to fail.

A couple of controllers missing work can have a big impact at a small airport already operating with limited tower staffing, he said.

"It's not like we have other controllers that can suddenly come to that facility and staff them. There's not enough people there," Daniels said Tuesday. "There's no overtime, and you have to be certified in that facility."

Air travel complications are likely to expand once a regularly scheduled payday arrives next week and air traffic controllers and TSA officers don't receive any money, the union leader said. If the impasse between Republican and Democratic lawmakers on reopening the government persists, the workers will come under more pressure as their personal bills come due, Daniels said.

"It's going to eventually be that when people don't have money, they have time to start making life choices and life decisions," he said.

Daniels said it's not realistic to expect controllers to stay focused only on their difficult jobs of keeping flights safe while they aren't getting paid even if they eventually should be entitled to back pay.

"That's not reality and it shouldn't be put on our backs," he said. "It's completely unfair that an air traffic controller is the one that holds the burden of 'see how long you can hang in there in order to allow this political process to play out?'"

Johnny Jones, secretary-treasurer of the American Federation of Government Employees chapter that represents TSA workers, said he was hearing questions like "How are we going to handle paying our bills?" and "How are we going to pay our child support? How are you going to pay for childcare? How are you going to pay your mortgage payments? If I have to miss work, am I going to get terminated?"

"The employees are struggling. They're assessing what they need to do and they're assessing how this is all going to work out," said Jones, who has worked as a screener since the TSA was established.

Some TSA officers already have called in sick, but Jones said he ddid not think the numbers were big enough to cause significant problems and delays at airports.

Aviation unions and U.S. airlines have called for the shutdown to end as soon as possible.

The unions are also making appeals to food banks, grocery chains and airports to secure support for workers during the shutdown. Hartsfield-Jackson Atlanta International Airport was offering federal workers $15 food vouchers and allowing them to park in the terminal, according to Jones.

John Tiliacos, the chief operating officer of Florida's Tampa International Airport, said the facility started preparing for the shutdown well before it began.

Nicknamed "Operation Bald Eagle 2" among airport staff, the efforts center around pulling together resources for the roughly 11,000 federal employees who are working at the airport without pay, including security screeners and air traffic controllers.

Tiliacos said the help would include a food pantry, free bus rides to work and a program with the local utility provider to keep the lights on at the homes of the workers.

"Whatever we can do to make life a little easier for these federal employees that allows them to continue coming to work and focus on keeping our airport operational, that's what we're prepared to do," he said.

Tampa's airport provided similar resources for impacted workers when the government closed for 35 days in 2018 and 2019 during President Donald Trump's first term. Tiliacos said he thinks the assistance helped prevent the widespread staffing shortages that other airports experienced during that shutdown. ,