A shocking video has emerged of the moment a US prison guard violently attacked a seemingly prostrate inmate in Louisiana, viciously setting his dog on him.

The CCTV footage, which was recorded in December 2012, shows prisoner Marcus Robicheaux being tackled to the ground by Iberia Parish Jail deputy David Prejean.

Despite Robicheaux being face-down on the ground, deputy Prejean appears to set his dog on the prisoner, who mauls him repeatedly. As this abuse continues, the deputy starts hitting, punching and eventually appearing to stomp on Robicheaux.

Warning: Video contains scenes that may be upsetting to some viewers

“I couldn’t do nothing but scream,” Robicheaux, who was in jail awaiting drugs charges, told The Advocate, adding that he remains unsure why he was singled out during the routine contraband search.

During the horrific attack, recorded over three minutes, three other guards are present. One unidentified officer – also holding a dog – stands closely by, while two watch from a distance.

As the attack unfolds the other prisoners remain stood with their foreheads against the wall or appearing to have their hair cut or shaved.

The footage came to light when it was subpoenaed by a federal grand jury as part of an investigation into abuse at the jail in the southern American state.