The top three U.S. airlines have told their flight attendants not to force passengers to comply with a new policy requiring face coverings, just encourage them to do so, according to employee policies reviewed by Reuters.

American Airlines Holdings Inc., Delta Air Lines Inc. and United Airlines Holdings Inc. have told employees that they may deny boarding at the gate to anyone not wearing a face covering, and are providing masks to passengers who do not have them, the three carriers told Reuters.

Inside the plane, enforcement becomes more difficult.

"Once on board and off the gate, the face covering policy becomes more lenient. The flight attendant's role is informational, not enforcement, with respect to the face covering policy," American told its pilots in a message seen by Reuters explaining its policy, which went into effect on Monday.

"Bottom line to the pilots: a passenger on board your aircraft who is being compliant with the exception of wearing a face covering is NOT considered disruptive enough to trigger a Threat Level 1 response," referring to some kind of intentional disruption by a passenger that could cause the captain to divert the flight.