The largest U.S. airlines say they are ready to crack down on passengers who refuse to wear face masks on board, a month after face mask requirements began and were ignored by many passengers.

The trade group representing carriers including American, Southwest, United, Delta, JetBlue and Alaska Airlines said they will increase the number of warnings made to passengers amid attempts to slow the spread of COVID-19 and make people more comfortable flying again.

The trade group, Airlines for America, also said airlines will step up penalties for those who refuse to comply. Chicago-based United Airlines said "any passenger that does not comply when onboard a United flight will be placed on an internal travel restriction list."

The Centers for Disease Control and Prevention has listed the use of cloth face masks and coverings as a way to slow the spread of COVID-19, which has caused thousands of deaths and ravaged the economy, including the airline industry. Three months after the pandemic started in the U.S., airlines are still flying fewer than a fifth of the passengers they did a year ago.

Airlines have struggled with how to vigorously enforce face-covering requirements since they went into effect a little over a month ago. Airline crew members could deny passengers the ability to board a plane if they weren't wearing face masks, but were cautioned not to enforce the rule too strictly once on board. There were exceptions for those with medical limitations, and passengers could also take them off to eat or drink.

"Southwest flight attendants will encourage adherence to the policy onboard the aircraft and are trained to handle a variety of scenarios that may occur during any flight," said Southwest Airlines spokesman Brad Parrish.

American has advised flight attendants to discreetly address a passenger not wearing a face mask and not covered by exceptions that include infants and people with particular medical needs, according to a message to members of the Association of Professional Flight Attendants. Cabin personnel should encourage compliance and seek to de-escalate any issues with other passengers frustrated over the situation, the union said.

But there have been complaints on social media that passengers aren't following the face mask rules.

Groups such as the Allied Pilots Association, which represents about 15,000 American Airlines pilots, have complained that they have no ability to enforce face mask rules. The union wants federal officials to make it a law.

Some members of Congress are pushing to make mask requirements a law, too.

"U.S. airlines are very serious about requiring face coverings on their flights," said Airlines for America President and CEO Nicholas E. Calio in a prepared statement. "Carriers are stepping up enforcement of face coverings and implementing substantial consequences for those who do not comply with the rules. Face coverings are one of several public health measures recommended by the CDC as an important layer of protection for passengers and customer-facing employees."

Airlines have been scrambling to assure customers that it is safe to fly again by touting more rigorous cleaning standards, air filtering systems and requiring employees to wear masks and gloves. But they are at a loss to stop passengers without diverting a flight or alienating customers.

American and Southwest have cautioned crew members not to divert planes if passengers won't wear a mask. Airlines including American said they will provide masks to customers who don't bring their own.

The airline trade group has also been pushing the Department of Homeland Security to start checking the temperature of passengers at Transportation Security Administration checkpoints, something the agency is considering, Secretary Chad Wolf said last month.