One of the men accused in the fatal gang rape of a student on a bus in New Delhi in December was found dead in his prison cell on Monday, prompting fury from the victim's family and accusations of murder.

Officials at maximum-security Tihar Prison in the Indian capital said 34-year-old bus driver Ram Singh had hanged himself with a makeshift noose.

Indian authorities have launched a major probe into the "mysterious" death of Singh.

"The Home Ministry has sought a report from the Delhi Police, and there is already an inquiry by the Tihar Jail. We are looking into the entire matter," Indian Minister of State for Home Affairs R.P.N. Singh told the media.

But Singh's parents said there was no way their son could have committed suicide and he had told them his life was in danger, while his lawyer said the death should be treated as murder, AFP reported.

"My son has not committed suicide. I had met him six days back at the court. He had told me that his life is in danger. This is definitely a murder," Mange Lal Singh, the father of Ram Singh, told the media.

Ram Singh's father said because one of his son's hands was badly injured in an accident several years ago, "he could not have committed suicide with just one hand".

Singh's lawyer said his client had been composed and calm when he spoke to him on Friday and that there were other inmates in his cell, raising questions about whether it was a suicide and how it could have gone unnoticed by staff in India's highest security prison.

The lawyer and a prison official said Singh had not been on suicide watch.

Police have described Singh as the ringleader of the five men and one juvenile on trial for the Dec 16 attack on the 23-year-old trainee physiotherapist in the Indian capital. The assault triggered nationwide protests, a toughening of rape laws and an intense debate about rampant crime against women in India.

All six accused have pleaded not guilty to rape and murder.

Tihar prison authorities ordered a magisterial inquiry after Singh was found dead in his cell at around 5 am, prison spokesman Sunil Gupta said.

AFP - Xinhua - Reuters