American Airlines said in a statement Friday that it will resume booking flights to capacity beginning July 1. The airline, which had previously limited capacity, said it will continue to notify customers of full flights and let them change flights at no cost. They'll also let passengers change seats on the plane provided there is room and they stay in the same cabin.
Delta and Southwest previously announced that they will block middle seats through the end of September. Alaska Airlines is blocking middle seats through the end of July.
United, meanwhile, told ABC News it does not "block middle and/or adjacent seats" and does not have a cap on capacity. United said it also offers rebooking options and notifications to customers if they "expect a flight to be more full."
"So far very few customers have rebooked," United said.
Leading U.S. airlines implemented rules last month requiring customers to wear face coverings on flights. In June, they vowed to step up enforcement after several instances in which people complained about other passengers not covering their faces.
Most of the airlines make exceptions for young children, for passengers while they are eating or drinking, and for those with some medical conditions.
The Federal Aviation Administration has declined requests by airlines and their labor unions to make masks mandatory.
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The Associated Press contributed to this report.