Content warning - Some viewers may find this footage disturbing

UPDATED: 7.38pm A 17-year-old 180 kilogram gorilla at Cincinnati Zoo has been shot and killed after a four-year-old boy fell into its enclosure.

Panicked zoo visitors watched helplessly and shouted, "Stay calm!" while one woman yelled, "Mommy loves you!" as Harambe loomed over the boy who had fallen into a shallow moat.

Zoo Director Thane Maynard said the gorilla didn't appear to be attacking the child but was "an extremely strong" animal in an agitated situation.

He said tranquilising the gorilla wouldn't have knocked it out immediately, leaving the boy in danger and the zoo has described the animal's death as tragic.

Independent zoo consultant Peter Stroud told Larry Williams the zoo did the right thing.

"Seeing the way he actually drag the child by the leg quite violently, and over quite some distance, the child was clearly in danger and I felt that the child's life just had to be put first."

Mr Stroud said when gorillas are tranquillized the effect is not instant.

"The animal is likely to b distressed at least in the seconds before it goes under the sedative or anaesthetic that's being used, and during that time there's a possibility the animal could well have turned on the child."