UN Secretary-General Ban Ki-moon on Saturday voiced his "deep sorrow" at the death of a Delhi female student, who was gang raped in a moving bus earlier this month in New Delhi, the Indian capital, saying that "violence against women must never be accepted".

"The secretary-general expresses deep sorrow at the death of the 23-year-old Delhi student who was gang raped by six men in a moving bus in New Delhi on Dec 16," said a statement issued on Saturday night by Ban's spokesman.

The secretary-general "offers his sincerest condolences to her parents, family and friends, and utterly condemns this brutal crime", it said.

"Violence against women must never be accepted, never excused, never tolerated. Every girl and woman has the right to be respected, valued and protected."

The Indian victim of gang rape, identified as Nirbhaya by local media, died at a private hospital in Singapore early on Saturday, the hospital said.

Kelvin Loh, chief executive officer of the Mount Elizabeth Hospital, said the patient died at the hospital at 4:45 am on Saturday after suffering from severe organ failure.

"The secretary-general welcomes the efforts of the government of India to take urgent action and calls for further steps and reforms to deter such crimes and bring perpetrators to justice," the statement said.

In apparent haste to calm public anger over the death of the medical student, Indian police on Saturday invoked murder charges against the six men involved in the gang rape.

"We hope to file the charge sheet by the Jan 3, 2013. Section 302 of IPC, which is the penal section for murder, has been added in the case," Dharmendra Kumar, special commissioner of police (law and order), told the media.

The secretary-general "also encourages the government of India to strengthen critical services for rape victims," the statement said. "UN Women and other parts of the United Nations stand ready to support such reform efforts with technical expertise and other support as required."

Also on Saturday night, UN Women, the main UN body dedicated to the protection of rights of women and girls, mourned the death of the Indian gang rape victim.

"UN Women joins the UN secretary-general in mourning the death of a 23-year-old student gang raped by six men earlier this month in New Delhi, India," the UN agency said in its official website. "UN Women and other parts of the UN stand ready to support reform efforts to deter such crimes and bring perpetrators to justice."