A group of U.S. senators is urging Delta Air Lines Inc and JetBlue Airways Corp to immediately reverse their decisions to reduce employees' hours, calling them contrary to the requirements of taxpayer-funded payroll assistance.

Delta and JetBlue have already received a portion of $25 billion in CARES Act money meant to protect airline workers' jobs and pay rates until Sept. 30 as the industry weathers a severe business slump during the coronavirus crisis.

Delta is set to receive $5.4 billion and JetBlue $935 million.

"You should not take one penny more of bailout funds unless you are prepared to protect your workers' jobs, pay and benefits," 13 senators including Elizabeth Warren and Kamala Harris, all Democrats, wrote to the airlines in two letters.

They said cutting employee hours was potentially illegal.

"Delta's work hour reductions, which comply with the CARES Act, ultimately protect jobs," Delta said in a statement.

JetBlue spokesman Philip Stewart said the move was in full compliance, adding that with flights to many cities suspended or significantly reduced, "there are quite literally no hours for our crewmembers to work in many cases."

With airlines bleeding cash, Stewart said payroll aid falls short of total payroll needs and that with little new revenue, JetBlue needs to make it last until Sept. 30 so it can preserve as many jobs as possible in October.

United Airlines Holdings Inc rolled back its plan to cut working hours for thousands of union aircraft and passenger service workers after their union filed a lawsuit seeking a halt to the plan.

"You should do the same," the letters said.

Sito Pantoja of the International Association of Machinists and Aerospace said: "The undeniable result is workers are involuntarily taking home less money to support their families. A pay cut is a pay cut."