New York (CNN) United will require flight attendants to wear face coverings or masks while they're on duty starting April 24th, according to an employee memo the airline shared with CNN.

The carrier, which says it's the first major US carrier to adopt the requirement, says it will begin with 20 masks onboard each aircraft serving domestic routes and 40 on international ones.

United will not replenish the supply after every flight--rather, it will do so " as needed and as supplies permit," said the airline.

Flight attendants also will have the option of to wear their own face coverings or use the ones United provides.

United noted that the requirement "is in line" with CDC recommendations that people wear face coverings when social distancing isn't possible.

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