The Transportation Security Administration announced Friday that it will issue a partial paycheck to most employees as thousands of other federal workers are set to miss a second biweekly paycheck, marking nearly a month since they were last paid.

TSA Administrator David P. Pekoske said the agency scraped together limited funds from varying sources in order to pay a portion of their workforce.

Employees "at the I-band and below," who worked in an exempted status for any amount of time from Dec. 23 to Jan. 5 will receive 40 hours of pay "to alleviate some financial strain," the TSA said. Employees in pay bands J and above, which are senior level positions with higher pay rates, will not receive a partial payment Friday.

"Challenging times require challenging decisions. While our aim is to pay everyone for their work, during this period of lapse in funding and because of legal and financial constraints, we needed to make decisions on how best to support the workforce using the limited funds we had," the agency said in statement to NBC News. "As such, it was determined that where we could, we would provide partial pay to employees and we focused on employees in the lower pay bands to help alleviate the financial strain they may be experiencing."

However, the TSA said Federal Air Marshals (FAMs), of all pay bands, will not receive a partial payment. The agency the FAMs account didn't have sufficient funding available to provide "meaningful partial payment" for Air Marshals. Instead, the TSA will issue 2018 performance awards for all FAMs "at the I-band and below."

Pekoske acknowledged in a tweet Friday morning that "anything short of full paychecks are a partial measure, and in no way compensates the #TSA workforce for the financial burden many are experiencing." But, he added, whenever funding is restored, "pay and travel reimbursement processing will be our highest priority."

The money for the TSA payments came from a combination of unlapsed FY2018 funds the agency had in "certain specific accounts" and the reallocation of some funds from other accounts, the agency said.

The shutdown began three days before Christmas when President Donald Trump and top Democratic leaders in Congress reached a stalemate over Trump's demand for a $5.7 billion wall along the U.S.-Mexico border.

Unions that represent air traffic controllers, flight attendants and pilots are growing concerned about safety and security of its members and passengers with the shutdown well into its fifth week.

The presidents of the National Air Traffic Controllers Association, Air Line Pilots Association and Association of Flight Attendants cautioned in a join statement that the airline industry "cannot even calculate the level of risk currently at play, nor predict the point at which the entire system will break. It is unprecedented."

Federal workers say going without pay is grinding them down, and they're not sure how much longer they can take it.

"At work, the morale is really low," said Tyler Kennard, an air traffic controller in San Diego. "It's actually more stressful now with this government shutdown than it was when I was in a war zone in Iraq doing the same job."

The retired Marine, who got his start in air traffic control in 2005 when he was based at Marine Corp Base Camp Pendleton, told NBC San Diego that he and his wife are worried about how they will pay for gas, the mortgage, their daughter's braces and their 4-year-old son Tucker's hospital visits.

During his nearly decade and a half in the profession, he has been through three other government shutdowns but this is the first time he's missed a paycheck.

"This is the one that’s hit us the hardest 'cause this is the first time where it’s gone where we haven’t got paid," Kennard said.