Among those workers are 2,850 pilots and 6,920 flight attendants, the airline said.
The carrier previously notified 36,000 employees they faced potential job cuts, but was able to reduce that number through various voluntary buyout and early retirement programs. United said roughly 7,400 workers have taken voluntary buyouts or early retirements and thousands of others have agreed to long-term leaves of absence.
"Unfortunately, all of our efforts so far to cut costs, raise debt and introduce voluntary options have not been enough to avoid involuntary furloughs entirely," a letter to United employees said.
Prior to the COVID-19 pandemic, United had just under 100,000 employees.
American Airlines announced late last month it will furlough approximately 17,500 U.S. based employees -- including 1,600 pilots and 8,100 flight attendants. Delta Air Lines said it plans to furlough almost 2,000 pilots this fall.
Southwest Airlines and JetBlue Airways both said they are looking to avoid any layoffs, at least in the short term.
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Various airline executives have voiced their support for a "clean extension" of the CARES Act in order to avoid such layoffs. A group of 16 Senate Republicans and over 200 House members have also expressed their support for an extension of the payroll support program, but Congress has yet to reach an agreement.
ABC News' Mina Kaji contributed to this report