The dog was travelling with its owner, and her baby and young daughter, passenger June Lara said in a Facebook post.
A dog is dead after its owner was forced to store the pup inside the overhead bin of a plane by a flight attendant on a United Airlines flight. At the end of the flight, they learned the animal had died.
United Airlines said they take full responsibility for the incident.
"The passenger adamantly refused but the flight attendant went on with the instruction", passenger Maggie Gremminger wrote. In August of a year ago, a dog passed away in a plane's cargo hold while the flight was delayed on the tarmac for two hours, moths after the death of a giant rabbit following a United flight from London to Iowa in April. The airlines issued a statement taking "full responsibility" for the dog's in-flight death.
"Another reason I will never ever fly or support doing business with United Airlines!" Animal deaths on planes are rare, but typically occur when a pet is being shipped as cargo, not in the passenger cabin.
"We didn't know it was barking a cry for help", Ms Gremminger said.
"It was clearly a dog and while the customer was adamant about leaving it under the seat, the attendant pushed her to do so", Gremminger said in a tweet. "There was no movement as his family called his name", Lara wrote. "I am heart broken right now".
"The gentleman sitting next to me (in 24B at that time) looked at one another to confirm what we had just seen, both of us acknowledging it was clearly a dog- and feeling stunned that it was placed there".
"I sat behind the family of three and thought myself lucky - who doesn't when they get to sit near a puppy?" According to Department of Transportation statistics on animal incidents on USA carriers, United has the highest rate of incidents involving loss, injury or death of animals during air transportation, with 2.24 incidents every 10,000 animals transported.
The Humane Society of the USA says air travel can be risky for pets and especially unsafe for brachycephalic breeds - such as pugs or bulldogs, whose short nasal passages make them vulnerable to oxygen deprivation and heat stroke.