Federal regulators are seeking to place new limits on service animals in flight in an effort to ensure that such animals really are providing assistance to people with disabilities.

The U.S. Department of Transportation issued a proposed rule Wednesday severely curtailing the types of service animals that airlines must accommodate.

Chiefly, dogs would be the only type of service animal that must be allowed. And emotional support animals would no longer be considered service animals for the purpose of air travel.

The plan is “intended to ensure a safe and accessible air transportation system,” the Transportation Department said. “It addresses concerns raised by individuals with disabilities, airlines, flight attendants, airports, other aviation transportation stakeholders, and other members of the public, regarding service animals on aircraft.”

People with disabilities are allowed to bring service animals on planes under federal law, but the specific rules have remained vague.

In recent years, airlines have reported a big spike in the number of emotional support animals people have sought to bring on planes. And with that increase has come more incidents with such animals, they said, prompting multiple airlines to impose their own limits.

Delta Air Lines, for example, said problems with urination, defecation, biting and other aggression led it to implement a more restrictive policy in 2018. The carrier said at the time that besides dogs, it had customers who sought to bring comfort turkeys, gliding possums known as sugar gliders, snakes, spiders and other animals aboard.

With the new regulation, the Transportation Department noted that it “wants to ensure that individuals with disabilities can continue using their service animals while also reducing the likelihood that passengers wishing to travel with their pets on aircraft will be able to falsely claim their pets are service animals.”

The rule would define a service animal as “a dog that is individually trained to do work or perform tasks for the benefit of a qualified individual with a disability, including a physical, sensory, psychiatric, intellectual or other mental disability.”

Under the plan, airlines could require passengers traveling with service animals to complete forms developed by the Transportation Department attesting that the animal has good behavior, is in good health and, for long flights, is trained not to relieve itself or to do so in a sanitary manner.

Those traveling with service animals may be directed to check in an additional hour ahead of other passengers under the proposal. Meanwhile, airlines could limit passengers with disabilities from bringing more than two service animals and they could stipulate that such animals fit within the traveler’s foot space on the plane.

Airlines could not refuse service animals based on breed, but could bar those that display aggressive behavior or pose a health or safety threat, the Transportation Department said. Airlines can require that service animals be harnessed, leashed or otherwise controlled.

The proposed rule will be up for public comment for 60 days.