An American man died of a "cardiac event" in his backyard in South Carolina while burying a woman he strangled, local authorities said.

Officials responded to reports of an unresponsive man in the town of Trenton and found 60-year-old Joseph McKinnon dead, Edgefield County Sheriff Jody Rowland and County Coroner David Burnett said in a statement carried by local media.

"McKinnon had no signs of trauma and natural causes were suspected," the statement said.

"While investigating the death and making notifications to the next of kin, a second body was located in a freshly dug pit.

"Patricia Ruth Dent, 65, of the same address appeared to have died by foul play."

Ms Dent, who local media reported was in a relationship with McKinnon, was found bound and wrapped in rubbish bags in the pit.

Post-mortem examinations confirmed the cause of both deaths.

Evidence and witness statements indicated that McKinnon attacked Dent in their home.