A US teenager who confessed to strangling, cutting and stabbing a nine-year-old neighbour because she wanted to know how it felt to kill someone has been sentenced to life in prison with the possibility of parole.

Alyssa Bustamante, 18, had pleaded guilty to second-degree murder and armed criminal action in the October 2009 slaying 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 the lesser charges, she avoided a trial and the possibility of spending her life in an adult prison with no chance of release.

Bustamante was 15 when she confessed to strangling Elizabeth, repeatedly stabbing her in the chest and slicing the girl's throat. She led police to the shallow grave where she had concealed Elizabeth's body under a blanket of leaves in the woods.

The teenager's defence 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 Sergeant David Rice testified that the teenager told him "she wanted to know what it felt like" to kill someone.

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

"I strangled them and slit their throat and stabbed them now they're dead," Bustamante wrote in her diary, which was read in court by a handwriting expert. "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.