GRAPHIC CONTENT WARNING.
UA1284 passenger June Lara, who was on the flight from Houston to New York City, shared how Monday's incident unfolded on Facebook.
"Today, I boarded my first United Airlines flight. On my way, I saw a Frenchie that looked identical to my own precious Winston. He was with his family - a young girl, no older than 8, her toddler sibling and their mother," Ms Lara wrote.
"I sat behind the family of three and thought myself lucky - who doesn't when they get to sit near a puppy? However, the flight attendants of flight UA1284 felt that the innocent animal was better off crammed inside the overhead container without air and water.
"They INSISTED [sic] that the puppy be locked up for three hours without any kind of airflow. They assured the safety of the family's pet so wearily, the mother agreed."
After landing, Ms Lara said the family opened his kennel but there was "no movement".
"I held her baby as the mother attempted to resuscitate their 10 month old [sic] puppy. I cried with them three minutes later as she sobbed over his lifeless body. My heart broke with theirs as I realised he was gone," she wrote.
Another passenger Maggie Gremminger also said she witnessed a United flight attendant instruct a passenger to place her dog carrier with the dog inside in the overhead compartment.
"The passenger adamantly refused but the flight attendant went on with the instruction," Ms Gremminger wrote on Twitter.
"At the end of the flight – the dog was found dead in the carrier. I am heart broken [sic] right now."
An United Airlines spokesperson told US's ABC News and BuzzFeed they assumed "full responsibility for this tragedy".
"[We] express our deepest condolences to the family and are committed to supporting them," the spokesperson said.
The spokesperson added they would be "thoroughly investigating" the incident to ensure it would not happen again.
SBS News has contacted United Airlines for further comment.
