A young Islamic State supporter has allegedly attacked a former Australian soldier inside a NSW prison cell.

The teenager allegedly used a sharp object to carve “e4e” into the man’s head, an apparent reference to the terror group’s “eye for an eye, tooth for a tooth” mantra, inside Kempsey prison on Thursday.

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The 40-year-old victim is said to be fighting for his life, while the teen has been charged with causing grievous bodily harm and moved out of the prison’s maximum security section.

Newscorp reports the former soldier served in East Timor.

Senior prison sources said the 18-year-old attacker was a known supporter of the terrorist group and had been previously caught sending graphic images of beheadings via internal mail to other ISIS extremists housed in Goulburn’s Supermax.

Another inmate and a guard were also assaulted at the Kempsey jail yesterday.

It’s understood the alleged attacker was also found to have a hand-drawn ISIS flag inside his cell, as well as having carved one into the wall.

The pair were in the same cell for just a few hours before Thursday’s attack.

With AAP