Dog accidentally sent through airport X-ray machine after being left in backpack at US airport

The dog was found in a backpack.
The dog was found in a backpack. Photo credit: Twitter//TSA Great Lakes

A dog was accidentally sent through an X-ray machine at a US airport after it was left in a backpack.

As a result, the USA's Transportation Security Administration (TSA) has urged passengers to triple-check their bags before boarding planes.

The incident happened at Dane County Regional Airport in Madison, Wisconsin this week.

"When traveling with any animal, notify your airline & know their rules," the TSA Great Lakes Twitter account tweeted on Wednesday (local time).

"At the checkpoint, remove your pet from the bag and send all items, including the empty carrier, to be screened in the machine."

In another Tweet, the TSA shared a video of how to properly board with your pets.

"If you think your pet will attempt an escape, ask to speak with a supervisor before removing the animal," TSA Great Lakes Tweeted.

Several people commented on the original post, claiming the passenger put their dog in the backpack to avoid paying the fee for travelling with their animal.

"That is inhumane to carry a pet in a sealed backpack, which of course is not airline approved for carrying a pet. Most likely they were trying to get past not having to pay the carry-on pet fee. Sick," one person tweeted.

"Probably put the dog in [a] backpack to sneak on the plane without paying pet fee," another tweeted.

According to the American Airlines and United Airlines websites, they charge $US125 (NZ$197) for a person to board with their pet on a one-way trip.