United Airlines is receiving a lot of criticism on Twitter after a photo of a dog left out on to the tarmac was circulated by a passenger, who claims the dog was left there for half an hour in the rain. United Airlines responded to the photo of the dog on the tarmac, but did not necessarily apologize.

According to the Daily Mail, passenger Barbara Galletly posted a photo of the pet dog on Twitter; the picture was taken through a plane window covered with raindrops. The dog crate is seen alone on the wet tarmac in Houston, Texas.

"Evil United Airlines leaves dog on rainy cold runway for more than half an hour despite alerts to staff," Galletly tweeted.

The photo has been re-tweeted 1,700 times.

Evil United Airlines leaves dog on rainy cold runway for more than a half hour despite alerts to staff :-((((( boo pic.twitter.com/xcwVOxIpV9

— barbara (@theregoesbabs) December 28, 2014

A United Airlines employee responded on Twitter, writing: "Thanks for telling us. The weather can change & may have been clear when PetSafe dropped off. They're the ones to call too."

Not exactly an apology for that poor dog. According to the Daily Mail, PetSafe is the airline's "specially-designed program" for transporting animals that are not eligible to travel in the aircraft cabin.

Now, how an airline - @United - should NOT treat dogs or the bipeds who love them. The originals are RT'd next pic.twitter.com/GtrZsIdUNE — Keith Olbermann (@KeithOlbermann) December 28, 2014

Twitter user Jason Vendel wrote in a response to United Airline's tweet: "No excuse. They are your contractor. Plus, she said staff was alerted. Just apologize and hope that dog is ok."

User name Karah added: "Award for worst use of Twitter today goes to United about concerns for a dog left on the tarmac in the rain."

Even famous "Chandelier" singer Sia caught wind of the issue: "I will never fly @United again. Thanks @theregoesbabs for exposing their sh**** treatment of our beloved pets," she tweeted.

A United Airlines spokesman told MailOnline Travel that the "angle of the photo on Twitter doesn't adequately tell what happened."

He said: "We checked several cameras and confirmed that our ramp employees in Houston never left the pet unattended after our PetSafe employees dropped the kennel off when we were ready to begin loading the aircraft for departure.

"As is our standard practice, our employees placed the pet kennel completely under the wing to protect the pet from the falling rain and onto the aircraft after completing the baggage loading."

Animals are loaded onto aircraft last so they can be unloaded first thing when the plane lands, he added.

As for the length of time the dog was on the tarmac, the spokesman said "it could have been about half an hour," adding that the temperature was about 50 degrees Fahrenheit at the time.