An 11-year-old boy could be tried as an adult in a US court and face a mandatory life sentence if found guilty of killing his father's pregnant fiancée, according to local media.

A judge in Lawrence County, western Pennsylvania, ruled that the child, Jordan Brown, would not be tried as a minor, the Pittsburgh Post-Gazette reported.

The paper reported that Master Brown sat silently with his hands and feet shackled during his preliminary hearing, before being returned to a juvenile detention centre.

He is accused of last month's shooting of Kenzie Houk, 26, in the back of the head while she slept, killing her and her unborn child who died of oxygen deprivation.