Two 12-year-old girls in the US have been charged as adults with attempted murder after authorities said they lured a classmate into woods and stabbed her 19 times in an attack inspired by a horror website.

The girls had allegedly planned the attack months earlier to please Slender Man, a fictional villain featured on the CreepyPasta site.

The girls wanted to become "proxies" for Slender Man, according to Fox news site, citing court papers.

According to the indictment, posted to the site, one of the two girls said they wanted to kill someone to show their devotion to Slender Man.

The victim, a fellow 12-year-old girl, was invited to stay over at the home of one of the accused on Saturday in Waukesha, Wisconsin.

She was lured into the woods and stabbed 19 times, with one cut landing just beside an artery, before being abandoned.

A cyclist found her and helped her get to hospital, where she is in a stable condition.

During a press conference, the Waukesha police chief alerted parents to the dangers of the internet.

"This should be a wake-up call for parents. Parents are strongly encouraged to restrict and monitor their children's internet usage," Chief Russell Jack said.

The two suspects, who are being held in a Wisconsin jail, are set to have their next court appearance on 11 June.