Image copyright Police Photo Image caption Morgan Geyser and Anissa Weier, both 13, are being tried in an adult court

Two 13-year-old girls accused of stabbing a classmate to please the online horror character Slender Man have pleaded not guilty in court.

Morgan Geyser and Anissa Weier have been charged with attempted murder over the attack.

Last week, a Wisconsin judge decided that they should be tried as adults, meaning they could face decades in jail if convicted.

The victim, 12 at the time, was stabbed 19 times but survived.

She was found by a cyclist crawling from the woods where she was attacked with stab wounds to her arms, legs and torso, after the attack in May 2014.

A judge entered not guilty pleas on behalf of both girls at a hearing on Friday.

Investigators say the two girls had been plotting for months to kill their victim in "dedication" to Slender Man, a fictional horror website character.

Image copyright ABC Image caption The Slender Man character is often the subject of online horror stories written by teens

They spoke of their desire to become the paranormal figure's "proxies" by killing to demonstrate their loyalty, police said.

A lawyer for one of the girls told the AP news agency before the hearing that his client would enter a plea of not guilty by reason of mental disease or defect.

Their lawyers have previously argued for the case to be heard in juvenile court, saying both teenagers were suffering from mental illness.

Following the attack, the suspects were found walking near a local highway and a knife was found in one of their backpacks.

The victim has recovered and since returned to school.