JetBlue Airways aircrafts are pictured at departure gates at John F. Kennedy International Airport in New York.

Airlines are stepping up policies to ensure that passengers and employees wear face masks on board.

Starting Monday, all JetBlue Airways passengers will be required to wear a face covering, the strictest policy so far in the U.S. and one that follows the New York-based airline's requirement that flight attendants wear a mask or face covering while on duty.

"Wearing a face covering isn't about protecting yourself, it's about protecting those around you," said JetBlue President Joanna Geraghty. "This is the new flying etiquette."

American Airlines and Delta Air Lines said they will require thousands of employees to wear face masks and will provide masks for passengers.

Airline labor unions have repeatedly demanded stronger protections against coronavirus. While air travel in the U.S. is down some 95% from a year ago, flight crews have raised concerns about catching the virus on the job and have sought federally mandated procedures.

"All airlines should follow JetBlue's lead, including its efforts to fully communicate the change before it becomes effective next week so that Flight Attendants are not put in the position of being enforcers without information and backing from the airline," said Sara Nelson, president of the Association of Flight Attendants, which represents some 50,000 cabin crew members, including those at United and Spirit.

American said it will start handing out face masks and sanitizing wipes for passengers next month and require that its roughly 30,000 mainline and regional flight attendants wear masks while on duty.

Fort Worth, Texas-based American is also going to step up cleaning procedures to disinfect areas including tray tables, seat belt buckles, window shares and seat-back screens, galleys, jump seats and surfaces in the cockpit.

"Our team members on the front line are truly going above and beyond during this time," said Kurt Stache, American's senior vice president of customer experience. "Their commitment is nothing short of incredible and we will continue to work with them to make sure they have the equipment they need to feel safe."

Delta said it will require employees to wear masks or face coverings any time it's impossible to keep a distance of 6 feet between themselves and a customer or another employee, starting on Tuesday. It will reevaluate the policy by June 30.

"We are strongly encouraging our customers to wear masks and will make them available at ticket counters, gates and onboard airplanes," four Delta officials said in a staff memo, which was seen by CNBC.

United Airlines last week started mandating masks or other face coverings for flight attendants, following similar moves by JetBlue and budget carrier Frontier Airlines.