Horror as toddler mauled to death after mother balanced him on railing above wild dog enclosure at zoo

A two-year-old boy was mauled to death by pack of African wild dogs, after he fell from a railing his mother had put him on to view the animals at a zoo in Pittsburgh.

The boy, who was visiting the zoo with his mother and friends, fell about 14 feet into an exhibit that's home to a pack of African painted dogs.

Despite zookeepers firing darts, and a police officer opening fire on the animals, desperate attempts to rescue the boy failed and he was savaged by the animals.

The child, from Pleasant Hills, Pittsburgh, reportedly fell over the railing and off a mesh barrier where the wild animals are kept, at about 11.45am on Sunday.

His mother, 34, watched horrified as the dogs attacked the boy.

Reports are unclear as to whether the boy was killed by the fall or the dogs.

When the boy fell, other visitors immediately told staff members, who responded along with Pittsburgh police.

Zookeepers called off the dogs, and seven of them immediately went to a back building.

Three more eventually were drawn away from the boy, but the last dog wouldn't come into the building, and police had to shoot him a zoo official said.

"It's clear that the dogs did attack the child, but whether he died of the attack or the fall has yet to be determined," Barbara Baker, president and CEO of the Pittsburgh Zoo & PPG Aquarium, said.

The dogs are about as big as medium-sized domestic dogs, 2 to 2.5 feet high and 17 to 36 kg, according to the zoo.

African wild dogs are also known as cape hunting dogs, spotted dogs, and painted wolves.

They have large, rounded ears and dark brown circles around their eyes and are considered endangered.

Lt. Kevin Kraus of Pittsburgh police told the Pittsburgh Post-Gazette that the the boy's 34-year-old mother placed the child in a standing position on a wooden railing overlooking the enclosure.

"Almost immediately after that he lost his balance, fell down off the railing into the actual pit and he was immediately attacked by 11 dogs," Lt. Kraus said.

Out of the pack of ten dogs zookeepers managed to drive away seven, however the remaining animals could not be forced away.

The dogs normally live in a 1.5-acre exhibit called the Painted Dog Bush Camp that's part of a larger open area called the African Savanna, where elephants, lions and other animals can be seen.

Visitors walk onto a deck that is glassed on the sides, but open in front where the railing is located. Visitors can look out at the dogs below. The mesh barrier is below the railing.

In May, some of the dogs crawled under a fence and escaped into a part of the exhibit that's usually closed. The zoo was on lockdown for about an hour as a precaution.

Ten African painted dogs were born at the zoo in 2009, and their mother died of a ruptured uterus shortly after delivering the litter. Five of the pups survived.

The mortality rate for painted pups is 50 per cent, even when born in the wild to a healthy mother.

It was only the second litter to be hand-raised in captivity, along with one in the United Kingdom, zoo officials said at the time.