A teenager had his ear bitten off when he was attacked by two American Staffordshire terriers while walking his dog in Sydney's south west yesterday afternoon.

The 19-year-old was walking along Lehn Road, East Hills, about 3.20pm, when the dogs ran towards him.

When the teenager tried to save his own dog, the terriers began mauling his legs and he fell to the ground, police said.

Police said the two dogs then began biting his head and tore off his left ear.

Neighbours came to help the teenager, and the dogs ran away.

The teenager's ear was found and surgeons at Liverpool Hospital worked to reattach it, but a spokeswoman for the hospital could not confirm whether the operation was successful.

He was in a stable condition this morning.

The two dogs have been seized and their owner is assisting police.

Police originally said the dogs were pit bull terriers, but have since been given updated information on the breed.

Inspector David Firth, from Bankstown police, said police believed the dogs escaped from their yard.

"My understanding is they actually scaled the colourbond fence," Inspector Firth told ABC 702 this morning.

He said police were speaking to the owner, who had been of "great assistance" in giving information about the dogs.

"He has owned them both since they were puppies and, in consultation with the council, we will be looking at taking action against him."

Inspector Firth said the breed is not on the restricted dog list in NSW and the dogs had not been declared dangerous.

"There was not any history of them attacking anyone in the past." 

Residents in Lehn Road said they often saw the owner walking the dogs on leads.

In 2010-11, there were 252 reported attacks from American Staffordshire terriers in NSW, and 16,503 of the dogs were registered with councils, according to local council data.

Attacks are defined as incidents where dogs rush at, bite, harrass or chase a person or another animal.

The five breeds that were responsible for the highest number of attacks in that period were the bull terrier (Staffordshire), Australian cattle dog, German shepherd, American Staffordshire terrier and rottweiler.

However, the dogs with the highest rates of attack were the Tibetan mastiff, pit bull terrier, Chesapeake Bay retriever, central Asian shepherd dog and mastiff.

Veterinarian Peter Higgins, of Dogs NSW, said attacks always came down to owners, not breeds.

"I'm getting to the stage now where I think we've got to make it compulsory for people to do training.

"Dogs are attack animals, so if you take control, if you’re the boss, then a lot of these attacks will actually go away."

Dr Higgins said the breed of dog believed to be involved in the attack needed a strict owner.

"They need a strong boss. If you’re not that sort of person, they’ll start doing their own thing.

"If it does have a strong owner it's fine, I’ve see it curled up in beds with kids, and on picnics and things like that.

"That’s because the owners know what the dogs are all about and the dogs turn into a sook basically, they turn into a baby."

Dr Higgins said there should be laws like those in Victoria to stop people going "underground" and cross-breeding dogs in order to get around breed restrictions.

"We say the laws have got to look at individual dogs, looking at breeds is a waste of time."