An alligator has attacked a woman who was walking her dog on a US resort island, pulling her underwater in a lagoon and killing her, authorities in South Carolina said.

Beaufort County Sheriff’s Office said the attack happened around 9.30am on Monday at the Sea Pines resort on Hilton Head Island.

Witnesses said the woman was pulled under the surface of the water by an alligator thought to be about 8ft long.

Police said the coroner had identified the woman as 45-year-old Cassandra Cline.

The dog did not appear to have been hurt, the sheriff’s department said.

The alligator believed to have been responsible for the attack was located later, the sheriff’s office said without specifying what happened after it was found.

The resort said it is working with authorities “to ensure necessary access to the site while the investigation is under way”.

“We are extremely saddened by this news and will share information with the community as it is made available,” Sea Pines said in a statement.