United Airlines is tightening rules on animals which can be brought into the cabin starting Monday. One new restriction is regarding the length of flights.

The airline said emotional support animals won’t be allowed on flights longer than eight hours.

“We have seen increases in onboard incidents on longer flights involving these animals, many of which are unaccustomed to spending an extended amount of time in the cabin of an aircraft,” the company said in a blog post.

Kittens and puppies under four months old will not be allowed on any flight regardless of whether they are emotional support animals, service animals, or pets. The airline said animals under four months typically don’t have the required vaccinations.

Only dogs and cats will be allowed as emotional support animals. Service animals will be limited to dogs, cats, and miniature horses.

The airline said reservations made before January 3, which is when the change was announced, will be honored.