Masks or other face coverings on Friday became mandatory for United Airlines' flight attendants, a measure aimed at stopping the spread of the coronavirus. The flight attendants' labor union is urging federal officials to require them for passengers, too.

The airline said it will provide flight attendants with surgical masks but they are also allowed to wear their own cloth masks.

"We understand that many aspects of the flight attendant's duties, both on and off the aircraft, can make practicing social distancing challenging, which is why this new initiative is so important," United said.

While air travel has plunged 95% because of the virus and stay-at-home orders, airlines are taking extra precautions to stop it from spreading on board. Executives have said passengers will fly once they feel safe and carriers have touted more rigorous aircraft cleaning and disinfection as well as blocking middle seats.

The Association of Flight Attendants, which represents some 50,000 cabin crew members including those at United, asked federal authorities to require that travelers as well as crews be required to wear masks to reduce the risk of spreading the disease.

"As some of the most frequent travelers, flight attendants feel a deep responsibility to ensure that our workplace risks of acquiring and spreading communicable diseases are minimized as much as possible," AFA's president, Sara Nelson, said in a letter Thursday to Transportation Secretary Elaine Chao and Health and Human Services Secretary Alex Azar.

The Department of Transportation and the Health and Human Services Department didn't immediately respond to a request for comment.

The union also asked that employers be required to provide personal protective equipment for airline staff and to halt all leisure travel, allowing just essential travel, until the pandemic is under control.

JetBlue Airways already requires crews to wear masks while on duty and says customers should do the same.

The Centers for Disease Control and Prevention "is recommending everyone wear a face covering in public, and we encourage our customers to follow these recommendations," a spokesman for the New York-based airline said.