A US woman was killed by an alligator while walking her dog near a lagoon bordering a golf course, police said, adding the animal guarded her body and kept rescuers back.

The 69-year-old resident of Hilton Head Island in South Carolina was found Tuesday at the edge of the lagoon, the Beaufort County sheriff's office said.

"Rescue efforts were made and an alligator appeared and was guarding the woman, interrupting emergency efforts," the office said in a statement.

It said it was the second fatal alligator attack in the county in less than a year.

Alligators are common in the southeast US, especially in Florida.

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