A woman has been killed by an alligator while trying to protect her dog in the US.

Cassandra Cline was attacked and pulled underwater by the alligator as she walked her border collie by a golf course lagoon in South Carolina

The 45-year-old was staying at a holiday home across the road from the course with her husband.

"Apparently the dog got too close to the lagoon and the alligator attempted to attack the dog," said Edward Allen, Beaufort County Coroner.

"In her effort to save the dog, the alligator turned on her."

In a statement, the Beaufort County Sheriff’s Office said officers responding to reports of the attack found the dead women in the water and recovered her body.

A trail of dog treats was found, along with one of the woman's shoes and her hat.

Beaufort County coroner Ed Allen, back left, talks with other officers near where authorities say Cassandra Cline was dragged into a lagoon by an alligator and killed while trying to save her dog, 20-08-2018. Image: Drew Martin/AP/Press Association Images

South Carolina Department of Natural Resources spokesman David Lucas said the 8ft alligator was later found and killed.

"She was walking the dog near the lagoon and the alligator came out of the water and tried to get the dog," said Mr Lucas.

"The lady tried to rescue the dog and a maintenance worker ran over to help."

Police said the dog was unharmed in the incident.

Thomas DiMaio, who rents a house across the road from the lagoon, said he heard shrieks while he was in the shower but thought they were from a bird.

He went outside a half hour later to find a crowd of people.

"It's really sad," said Mr DiMaio.

"She didn't have any children. The dog was her child, I guess."

He said he would see Ms Cline and her dog walking two or three times a day.