Two chimpanzees who mauled an American student in South Africa will not be killed because authorities found the animals defended their territory against a trespasser, a park director said Tuesday.

Andrew Oberle, 26, lost an ear, several fingers and toes and a testicle in the attack on Thursday when he entered a restricted enclosure at Jane Goodall Institute Chimpanzee Eden in the northeast of the country, the national parks board found.

"They don't believe the chimpanzees should be put down because they were in defence of their territory and they did not escape or try to climb over the fence with the intent to hurt anyone or to attack any tourist," said "Chimp Eden" director Eugene Cussons.

Oberle, a Masters student in anthropology and primatology at the University of Texas at San Antonio, was doing research at Cussons' sanctuary for abused and orphaned chimps.

While taking tourists on a visit of the facility he climbed over an outer safety fence to a restricted zone in front of the 4.5-hectare encampment fence to fetch a stone that the apes might try to throw at people.

"Andrew was never trained to do so. There's only a few people that's trained at the highest level that actually go into the no-go zone," Cussons told AFP

Oberle stepped onto an outer wire mesh of the camp, which triggered the defensive instincts of the pack leader, Amadeus, and his second-in-command Nikki. They grabbed his one foot and dragged him down, pinning him under the fence.

"This fierce struggle that went on between him and the chimpanzees damaged that section of the fence and the two male chimpanzees used that damaged section to be able to crawl underneath and exit."

They then dragged him 30 meters (100 feet) away. Cussons was called to kill the animals if necessary when first rescue attempts did not work.

He fired two warning shots, then retreated to his car when one of the chimps charged.

"Nikki jumped onto the bonnet of the vehicle and started smashing the bonnet and the windshield trying to get into my vehicle."

"I fired a round through the windscreen and hit him in the abdomen."

As the ape retreated, Cussons used intimidation to regain control.

"I ran after him still pointing the firearm at him so he could understand that I was now the dominant presence."

He then stabilised Oberle until paramedics arrived.

"He was conscious, he was speaking to me. He was badly injured. He had deep lacerations. He had injuries to his feet and hands and lacerations all over his body," Cussons told AFP.

The student was being kept unconscious in hospital, said Cussons.

"Andrew is in a drug-induced coma for his own comfort. He is in a stable condition now."

His family has flown to the country from the United States.

Chimp Eden gained fame through an Animal Planet TV series called "Escape to Chimp Eden".

The sanctuary currently keeps 33 chimps, which do not naturally occur in South Africa, in three large camps.

Seven were in the third camp for worst abused chimpanzees, where the attack happened.

Amadeus was rescued from Angola when he was three. His parents were killed in the bush meat trade.

Nikki came all the way from Liberia on the other side of the continent when he was four. He had been kept as a pet.

Both apes are around 16 years old.

Nikki's gunshot wound was treated at the Johannesburg zoo and he was doing well, said Cussons.

"I had to make sure Andrew's life is protected. Once the chimp's behaviour was submissive there was no reason for me to hunt them down and kill them."