Tripoli - Libya's state prosecutor is seeking to identify guards shown in a video apparently beating a son of Muammar Gaddafi held in a Tripoli prison, his office said on Monday.

The video was posted a week after a Tripoli court sentenced another of Gaddafi's sons, Saif al-Islam, and eight former regime officials to death over crimes during the 2011 uprising that toppled his father, who was later captured and killed by rebels.

The video appears to show guards goading and beating Saadi Gaddafi, 42, who was extradited to Libya by Niger last year.

He has been held since then in a Tripoli prison facing charges over the killing of a soccer player when he headed the Libyan Football Federation, as well as other crimes.

In the video, Saadi is beaten on his face while being questioned and later on the soles of his feet, tied to a pipe, while he is screaming.

He is also forced to listen and watch other screaming prisoners being beaten outside the room he is held in.

The video's authenticity could be not be independently verified.

“The graphic video that seems to show prisoners being beaten raises serious concerns about the methods used to interrogate al-Saadi Gaddafi and other detainees at al-Hadba prison,” Joe Stork, deputy Middle East director at New York-based Human Rights Watch, said.

Tripoli's prosecutor general said in a statement he had launched an investigation to identify the guards and “to take the necessary legal action against them.”

Last week's court ruling against Saif al-Islam was passed in absentia since he has been held since 2011 by a former rebel group in Zintan, a western region beyond Tripoli's control.

Other former Gaddafi regime officials sentenced to die by firing squad included former intelligence chief Abdullah al-Senussi and ex-prime minister Baghdadi al-Mahmoudi.

Rights groups say the trial was riddled with legal flaws.

Libya is in chaos with two governments fighting each other, while Islamic State militants have gained a toehold in the growing security vacuum.

Reuters