A 23-year-old Indian student gang-raped on a bus in Delhi last week has died in a Singapore hospital.

Earlier, doctors treating the woman said her condition had deteriorated and there were indications she had severe organ failure.

The woman was brutally attacked by at least six men on a bus on December 16.

Police said she was raped for nearly an hour, beaten with an iron bar and thrown out of the moving vehicle.

She was transported to Singapore for medical treatment on Thursday and diagnosed with infections in her lungs and abdomen, as well as significant brain injury.

She had also suffered from cardiac arrest.

"We are very sad to report that the patient passed away peacefully at 4:45am on Dec 29. Her family and officials from the High Commission of India were by her side," Mount Elizabeth Hospital chief executive officer Kelvin Loh said in a statement.

"Despite all efforts by a team of eight specialists in Mount Elizabeth Hospital to keep her stable, her condition continued to deteriorate over these two days.

"She had suffered from severe organ failure following serious injuries to her body and brain.

"She was courageous in fighting for her life for so long against the odds but the trauma to her body was too severe for her to overcome."

Indian prime minister Manmohan Singh said he was deeply saddened by the death and the best way to honour her memory was with constructive action.

"I am deeply saddened to learn that the unfortunate victim of the brutal assault that took place on December 16 in New Delhi has succumbed to the grievous injuries she suffered following that attack," he said in a statement.

"We have already seen the emotions and energies this incident has generated. These are perfectly understandable reactions from a young India and an India that genuinely desires change."

The attack had sparked demonstrations across India, culminating last weekend in pitched battles between police and protesters outraged over the lack of safety for women in the capital.

Mr Singh's government has been battling criticism that it was tone-deaf to the outcry and heavy-handed in its response to the protests in Delhi.

Some Indian medical experts questioned the decision to airlift the woman to Singapore, calling it a risky manoeuvre given the seriousness of her injuries.

They said she was already receiving the best possible care in India, which possesses world-class medical facilities.

Six men are in custody in connection with the assault.

The rape victim has not been identified but some Indian media have called her "Amanat", an Urdu word meaning "treasure".

Most rapes and other sex crimes in India go unreported and offenders are rarely punished, women's rights activists say.

But the brutality of the assault on December 16 triggered public outrage and demands for both better policing and harsher punishment for rapists.

Delhi has the highest number of sex crimes among India's major cities, with a rape reported on average every 18 hours, according to police figures.

Government data show the number of reported rape cases in the country rose by nearly 17 per cent between 2007 and 2011.

ABC/wires