Child fell from a viewing platform into an enclosure holding wild African dogs and was mauled by a pack of the animals

A young boy has been killed at Pittsburgh zoo after he fell into an enclosure holding African wild dogs and was attacked by a pack of the animals.

Lieutenant Kevin Kraus of the Pittsburgh police said the attack happened at about 11.45am local time on Sunday after the mother picked the child up and put him on top of a railing at the edge of a viewing deck. "Almost immediately after that he lost his balance, fell down off the railing into the pit, and he was immediately attacked by 11 dogs," Kraus said. "It was very horrific."

It was not clear whether the boy died from the fall or the attack, said Barbara Baker, president of the zoo. Staff at the zoo at first estimated the boy fell 4.2 metres (14ft), but police said it was 3.3 metres.

Authorities said zoo staff and then police responded "within minutes" but visitors described that time as being filled with screams for help. Zookeepers called off some of the dogs, and seven of them immediately went to a back building. Three more eventually were drawn away from the child, but the last dog was aggressive and police had to shoot it.

Experts said the death was highly unusual. Steve Feldman, a spokesman for the US Association of Zoos and Aquariums, said no one he had spoken to could recall any deaths of children at an accredited zoo over the past 40 years or more. Feldman said Pittsburgh zoo successfully completed its five-year review in September, which meant it met or exceeded all safety standards.

Authorities did not release the name of the boy or his mother, but said she was 34 years old and lived in Pleasant Hills, just outside Pittsburgh. The boy's father arrived on the scene soon after the accident, police said.

The zoo was immediately closed, and it was not clear when it will reopen, authorities said.

The so-called painted dogs are about as big as medium-sized domestic dogs, and weigh 16 to 36kg (37 to 80lb), according to the zoo. They have large, rounded ears and dark brown circles around their eyes and are considered endangered.

The attack happened in a 0.6-hectare (1.5-acre) exhibit called the Painted Dog Bush Camp that was part of a larger open area holding elephants, lions and other animals. Visitors walk on to a deck that has glass on the sides, but is open in front with a railing about 1.2 metres high.

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

Past fatal attacks at US zoos have prompted a review of safety features. In 2007 a tiger jumped over a wall at the San Francisco zoo, killing one visitor and wounding two others. Authorities first said the wall was 5.5 metres, but a review found it was just 3.6.

In September a man jumped off an elevated viewing train at the Bronx zoo in New York and was severely mauled by tigers.

Kraus said there was nothing to prevent visitors to the painted dog exhibit from jumping into the exhibit area.

Police and the local medical examiner's office are investigating. They have not yet interviewed the mother or father, who are receiving grief counselling.

Baker said the zoo would also investigate. She said no decision had been made yet on the future of the exhibit.