Aymen Terkmani had been serving a minimum 33-year term for the murder and sexual assault of a teen. (Brendan Esposito/AAP PHOTOS)

A notorious murderer who battered a teenage boy to death after raping him has been killed while serving a long jail stint for the “brutal and horrific” attack.

Aymen Terkmani has died in the maximum-security Lithgow Correctional Centre after an assault on Wednesday that left the 31-year-old critically injured.

The inmate was given medical help, but paramedics declared him dead, Corrective Services NSW said in a statement on Thursday.

Prison authorities and NSW Police are investigating the incident.

Terkmani was serving a minimum 33-year sentence for the 2015 murder and sexual assault of 16-year-old Mahmoud Hrouk.

The teenager’s brother spotted his bloodied, half-naked body through the window of an abandoned home in Sydney’s Fairfield East the day after Mahmoud told his mother in a cut-off call that he was with his “friend Aymen”.

Terkmani’s sentencing judge, NSW Supreme Court Justice Lucy McCallum, said the then-21-year-old subjected the youth to “unspeakable violence” before he died.

“The offender subjected the victim to the most brutal and horrific attack, inflicting injuries too numerous to list and too gruesome to describe,” Justice McCallum said as she sentenced him to a maximum term of 45 years.

Prosecutors had called for Terkmani to receive a life sentence.