The pandemic could permanently alter the industry, United’s chief executive, Oscar Munoz, and president, Scott Kirby, warned employees in a memo on Friday. Also on Friday, Delta Air Line’s chief executive, Ed Bastian, asked his employees to consider taking unpaid leaves from the company.

“If the recovery is as slow as we fear, it means our airline and our work force will have to be smaller than it is today,” the United executives wrote in their memo.

The airline warned that it planned to cut service next month by more than 60 percent and still expected its flights to be largely empty. As a result, United is likely to have to cut its schedule even more in May and June.

Mr. Bastian, Delta’s chief executive, thanked the 21,000 Delta workers who have volunteered to take unpaid leave, but encouraged more to join them.

“While this assistance is welcome, it’s important to remember that the relief package is not a cure for the unprecedented challenges we face,” Mr. Bastian said.