A mother whose three-year-old son fell into a gorilla enclosure is heard pleading for help in a 911 call released by police.

The woman, identified in US media as Michelle Gregg, repeatedly tells her son to "be calm" during the incident at Cincinnati Zoo.

But her panic is evident when addressing the operator.

She is heard saying: "My son fell in with the gorilla. There's a male gorilla standing over him. I need someone to contact the zoo please."

She goes on: "He's grabbing my son. I can't watch this. I can't watch."

Another witness, Catie Goodrich, says in a call: "There's a baby in the zoo that fell in the gorilla moat. Hurry! Hurry! The gorillas are out. Oh my god."

Within nine minutes of the first call the zoo's dangerous animal response team shot and killed the 200kg gorilla to protect the boy.

The killing of Harambe prompted anger around the world, with the parents reportedly receiving death threats and more than 450,000 people signing a petition calling for them to be investigated.

A source told Cincinnati.com that police have completed their investigation and no charges will be filed.
The family said the toddler was "doing well" after the scare.

Meanwhile, the zoo has collected sperm samples from the 17-year-old gorilla - raising hopes he could have posthumous children.

Experts said it would more likely be used for research.