Transportation Security Agency officers forced to work without knowing when their next paycheck is coming are no longer just calling in sick. Now, 18 days into the partial government shutdown, some are resigning, according to Hydrick Thomas, who heads the TSA Council on the American Federation of Government Employees.

“Every day I’m getting calls from my members about their extreme financial hardships and need for a paycheck. Some of them have already quit and many are considering quitting the federal workforce because of this shutdown,” the official said in a statement posted to the union's website on Tuesday. He did not specify how many had quit.

As essential employees, the country's approximately 50,000 TSA officers must remain on the job despite the shutdown.

Thomas warned that the situation poses a "massive security risk for American travelers since we don't have enough trainees in the pipeline or the ability to process new hires."

“Our officers have undergone a tremendous amount of training and taken an oath to protect this country,” he added. “They are highly specialized in screening passengers and do so better than any private contractor, but we’re risking losing them by offering no pay for long hours and dangerous work...”

At the very least, Thomas said the shutdown will mean increased wait times for travelers going through security checkpoints.

Michael Bilello, TSA's assistant administrator for public affairs, disputed the union's claim that wait times and workers calling in sick were up significantly.

In a tweet Monday, he acknowledged that TSA sick calls were up slightly to 4.6 percent as of Jan. 7 compared to 3.8 percent the same time last year. But in a series of follow-up tweets, he touted short wait times at major airports throughout the country.

"Nationwide, @TSA screened just over 2 million passengers yesterday (Monday). 99.9 percent of passengers waited less than 30 minutes; 92.9 percent of passengers less than 15 minutes, Bilello said. "In TSA Precheck lanes, passengers on average waited less than five minutes."

But the TSA Council on the American Federation of Government Employees president J. David Cox says TSA is asking too much of its employees.

“TSA Officers already have the least amount of rights of any federal officer, some of the lowest pay and highest attrition rates in government, and among the lowest morale of any federal agency," he said. "Working for weeks on end without being compensated – while already being short-staffed – only makes their situation worse.”

USA TODAY has reached out to TSA representatives for comment.

More: Working without pay duringshutdown, more TSA agents are calling in sick