United Airlines Holdings (NASDAQ:UAL) said Tuesday it would ban passengers who refuse to wear face masks on flights, part of a broader effort by major airlines to crack down on travelers not taking safety measures to minimize the spread of COVID-19.

Beginning on June 18, United passengers who refuse to wear masks will be placed on an internal restricted list "for a duration of time to be determined pending a comprehensive incident review." The only exceptions to the policy are individuals with medical conditions that prevent them from wearing a mask, those who can't put on or remove a mask themselves, and small children.

A United 787 Dreamliner landing.

Image source: United Airlines.

"Every reputable heath institution says wearing a mask is one of the most effective things people can do to protect others from contracting COVID-19, especially in places like an aircraft where social distancing is a challenge," United chief customer officer Toby Enqvist said in a statement. "Today's announcement is an unmistakable signal that we're prepared to take serious steps, if necessary, to protect our customers and crew."

The announcement comes as Airlines for America, a trade group that represents United, Delta Air Lines (NYSE:DAL), American Airlines Group (NASDAQ:AAL), Southwest Airlines (NYSE:LUV), and three other carriers, said its members will be "vigorously enforcing" face covering policies.

The move is both designed to protect passengers and employees, and to reassure potential travelers. Airlines saw travel demand fall by as much as 90% in March as the pandemic spread, and even as they rebuild their schedules for July the carriers are still scheduling less than half the flights they flew one year ago.