A sickening gang-beating in Sydney's west in which the young victim was stomped and kicked in the head long after he lost consciousness has been posted online with the caption "Welcome to Mt Druitt".

While the motivation for the North Parade attack is not yet known, the youngster is quickly knocked out by the mob of at least a dozen, and is hit with what looks like a school chair.

As the boy lies bloodied and limp, he is kicked in the head several more times before one of the attackers squats over his body and screams "Greater West Sydney, mother---ker", before delivering yet another blow.

Only one among the mob appears to try and stop the violence, saying "leave him, leave him," but his words go unheeded.

It is not clear when the video was filmed but police have confirmed they are investigating the shocking attack.