An American teenager who strangled and stabbed a nine-year-old neighbour because she wanted to know how it felt to kill someone has been jailed for life.

Alyssa Bustamante, 18, was 15 when she confessed to the murder of Elizabeth Olten in their small Missouri town. In her diary, she described the experience as "pretty enjoyable."

Bustamante had been charged with first-degree murder. By pleading guilty to lesser charges of second-degree murder she avoided a trial and the possibility of spending her life in an adult prison with no chance of release.

With her hands shackled to her waist and her feet shackled together, Bustamante rose and faced Elizabeth's mother before she was sentenced. She struggled to compose herself.

"I know words can never be enough and they can never adequately describe how horribly I feel for all of this," she said. "If I could give my life to get her back. I would. I'm sorry."

The teenager's lawyers had argued that Bustamante's use of the antidepressant Prozac had made her more prone to violence. They said she had suffered from depression for years and once attempted suicide by overdosing on painkillers.

But prosecutors noted that Bustamante had dug two graves several days in advance. Missouri State Highway Patrol Sgt. David Rice said that the teenager told him "she wanted to know what it felt like" to kill someone.

Prosecutors also cited diary entries in which Bustamante described the exhilaration of killing Elizabeth.

"I strangled them and slit their throat and stabbed them now they're dead," she wrote.

"I don't know how to feel atm. It was ahmazing. As soon as you get over the 'ohmygawd I can't do this' feeling, it's pretty enjoyable. I'm kinda nervous and shaky though right now. Kay, I gotta go to church now...lol." Bustamante then headed off to a youth dance at her church while a massive search began for the missing girl.