Transportation Security Administration workers at several major airports around the country, working without pay since the partial government shutdown began Dec. 22, have been calling in sick in heightened numbers, according to union and airport management officials.

More than 150 TSA employees, many of them responsible for screening passengers, called in this morning at Kennedy International Airport in New York to say they were ill or otherwise unable to work their shifts, according to a union official with knowledge of the situation.

The staffing gap was covered by other officers, who are also working without pay, the person said.

The so-called call-outs have spiked to three times their normal level at Dallas-Fort Worth International Airport, where an average of 25 TSA employees usually call in sick per shift, a local official of the agency told CNN, which first reported on the story.

The shutdown has left 800,000 federal workers either on furlough or working without pay because their jobs are deemed essential.

Dissatisfaction and anger have been growing over the past few days, as workers face their first period without a paycheck this weekend. Increased sick calls at TSA could increase the pressure on the White House and Congress to resolve the shutdown more quickly.

A federal official, who spoke on condition of anonymity because the official was not allowed to comment publicly on the matter, said the call-outs seemed to be a coordinated protest. But union officials said that many of the workers who called in sick were most likely seeking alternative temporary employment to make up for the lost wages.

A TSA spokesman played down the issue and attributed the increases in call-outs to typical patterns around the holidays and flu season. But he said that wait times for passengers could be longer if sick calls increase.

“Security effectiveness will not be compromised and performance standards will not change,” the spokesman, Michael Bilello, said in a text. “TSA is grateful to the agents who show up to work, remain focused on the mission and respectful to the traveling public as they continue the important work necessary to secure the nation’s transportation systems.”

Airport wait times have not yet increased at JFK, and operations have not been affected, according to officials with the Port Authority of New York and New Jersey, which runs the airport.