DENPASAR, Indonesia (REUTERS) - A US man confessed in an Indonesian court on Thursday to killing his pregnant girlfriend's mother, in a grisly case in which the victim's battered body was found in an abandoned suitcase on the resort island of Bali.

Tommy Schaefer and Heather Mack, both from Chicago, are being tried separately and have been charged with premeditated murder in the death of Shiela von Wiese-Mack.

Schaefer has argued self-defence, telling the court Wiese-Mack attacked him in anger because she objected to the couple's relationship.

"She squeezed my neck for about 20 to 30 seconds so I couldn't see because my eyes were watering," Schaefer said, describing through an interpreter the moments leading up to the murder.

"She wanted to reach for a bowl but I reached it before she did. I couldn't see so I just swung it."

Mack, who is eight months pregnant, told a court hearing on Wednesday that Schaefer had killed her mother after she had threatened to kill the couple's unborn baby, according to media.

Bali police conducted a four-month investigation into the killing that included a re-enactment with the defendants at the luxury hotel where Wiese-Mack's body was found.

Other evidence submitted to prosecutors included closed-circuit TV footage showing the couple speaking to a taxi driver after dropping the bloodied suitcase along with other luggage outside the hotel.

The pair could face the death penalty if found guilty of the murder, which took place in August of last year.