A service dog strolls through the isle inside a United Airlines plane at Newark Liberty International Airport while taking part in a training exercise, in Newark, N.J.

The number of emotional-support animals flying on U.S. airlines have surged in recent years, airlines have said, prompting concerns about allergies, soiled cabins and biting.

Last year, a passenger's emotional-support dog bit the face of the traveler in the next seat aboard a Delta flight.

Now the U.S. Department of Transportation (DOT) is seeking input from the public about how to prevent untrained animals from flying, and how to ensure that those with disabilities can best fly with their service animals.

The DOT said Wednesday that it will collect comments for 45 days, in an effort to potentially issue new regulations of such animals on board.

The three biggest U.S. airlines — American, Delta and United — have this year tightened rules for emotional-support animals as more and more passengers bring these animals on board. The carriers are requiring passengers to provide documents — including those that say the animals will be well-behaved, and give advance notice.

Under federal law, airlines must accept emotional-support animals in the cabin free of charge. The ease with which passengers can certify their animals for emotional support has raised concerns that some are bringing pets aboard to avoid airlines' in-cabin pet fees.

"Flight attendants and passengers have been bitten, attacked, and inconvenienced by animals who are not properly trained to be in a confined public environment," said Sara Nelson, president of the Association of Flight Attendants, a union that represents some 50,000 flight attendants at airlines including United and Alaska.

"This is interfering with the rights of our veterans and people with disabilities who legitimately need these animals to travel. DOT should, without delay, issue clear guidance on emotional support animals to standardize practices across the industry," she said.

The DOT is also giving airlines some wiggle room to limit the movement of some emotional-support animals in the cabin.

"The Enforcement Office will not take action against carriers that impose reasonable restrictions on the movement of (emotional-support animals) in the cabin so long as the reason for the restriction is concern for the safety of other passengers and crew," the DOT said.

"Such restrictions may include requiring, where appropriate for the animal's size, that the animal be placed in a pet carrier, the animal stay on the floor at the passenger's feet, or requiring the animal to be on a leash or tether."