The US Department of Transportation on Wednesday proposed that only specially trained dogs that qualify as service animals would be allowed in the cabin at no charge.

Airlines could ban emotional-support animals including untrained dogs, cats and more exotic companions such as pigs, pheasants, rabbits and snakes.

Airlines say the number of support animals has grown dramatically in recent years. They lobbied the Transportation Department to crack down on what they consider a scam - passengers who call their pets emotional-support animals to avoid pet fees that generally run more than $US100 ($A146) each way.

Flight attendants had pushed to rein in support animals, and they too were pleased.

"The days of Noah's Ark in the air are hopefully coming to an end," said Sara Nelson, president of the Association of Flight Attendants. She said some of her union's members were hurt by untrained pets.

Southwest Airlines handles more than 190,000 emotional support animals per year. American Airlines carried 155,790 emotional support animals in 2017, up 48 per cent from 2016, while the number of checked pets dropped 17 per cent. United Airlines carried 76,000 comfort animals in 2017.

Transportation Department officials said in a briefing with reporters that they proposed the changes to improve safety on flights. Some passengers have been bitten by support animals, and airlines complain that they relieve themselves on planes and in airports.

Australian Associated Press