Major US airlines announced Monday new health and safety measures to protect in-flight personnel and passengers from the coronavirus, even as states begin lifting restrictions.

In a memo to employees and shared with AFP, Delta Airlines said that from Tuesday, "all employees and partners" will be required to wear masks in case they cannot maintain social distancing.

Flights from all airlines have been grounded since mid-March as potential passengers hunkered down at home under lockdown orders to slow the spread of the virus. Most transatlantic flights were suspended after an order from the White House.

But over the past several days, certain US states -- including Georgia, Tennessee, Texas, Alaska and Colorado -- began easing their restrictions, raising fears of an increase in air passenger traffic in the near future.

Airlines have also tended to cancel multiple flights in recent weeks to rationalize costs, putting larger groups of passengers on the same aircraft.

Mandatory masks for cabin crew are in line with recommendations from the Centers for Disease Control and Prevention (CDC) to prevent transmission of COVID-19.

"We are also strongly encouraging our customers to wear masks and are making masks available at ticket counters, gates and onboard airplanes," a Delta spokeswoman told AFP.

American Airlines personnel on national and regional flights will also begin wearing masks from May 1, the company announced Monday.

In addition, the airline will offer protective gear to passengers.

"In early May, American will start the process of distributing sanitizing wipes or gels and face masks to customers," the aviation giant said in a statement.

"This offering will expand to all flights as supplies and operational conditions allow."

Fellow US flight company United Airlines said that while all personnel are required to wear masks on board, it is not mandatory for passengers.

Flights from all US airlines have been grounded since mid-March 2020 as potential passengers stay home in coronavirus lockdowns
Flights from all US airlines have been grounded since mid-March 2020 as potential passengers stay home in coronavirus lockdowns AFP / MANDEL NGAN

We "suggest customers follow the directives from their local officials and CDC guidance that recommends face coverings in places where social distancing is difficult," a spokesman for the airline told AFP.

According to the spokesman, the company is already "taking additional steps to promote social distancing in the air and on the ground."

The announcements come the day after Sara Nelson, the president of the Association of Flight Attendants-CWA, posted a tweet slamming the lax health measures on US flights.

Nelson tweeted a photo showing a plane packed with passengers who were not observing social distancing, many of whom also were not wearing masks.

"ENOUGH!" Nelson wrote. "This was TODAY on a four hour flight. This is not ok."

She urged the federal health and transportation departments to mandate that masks be worn in airports and on planes.

American Airlines announced that it also would be stepping up its cleaning and disinfection procedures from this week.

United began earlier in April to automatically assign seats on flights to help maintain social distancing, he said.

"We expect to implement additional, temporary changes to our policies on... seating throughout the entire aircraft, as well as making adjustments to the boarding process," the spokesman said.

In particular, United will limit advanced seat selection and board fewer passengers at a time to prevent any crowding as people take their seats.

The policies will remain in place until May 31.