India's Supreme Court on Monday upheld the death penalty for three men convicted in the gang rape of a young woman in Delhi in 2012, a landmark case that brought an unprecedented level of attention to violence against women in the country.

The brutal rape of the 23-year-old medical student, who died of her injuries, had sparked nationwide outrage and forced changes in the law. But six years later, there are few signs that sexual violence against women is abating.

In 2016, there were around 40,000 rapes reported in India - up 60 percent from 2012, according to government data, and more cases go unreported, activists say.

"Crimes against women will keep on rising unless the criminals are sent to the gallows," the father of the victim, who cannot be named under Indian law, told reporters after the Supreme Court ruling.

A three-member bench led by Chief Justice Dipak Misra dismissed petitions filed by the men to review a 2017 order by the top court, which had confirmed the death penalty given to them by the Delhi High Court.

"There is no merit in the petitions," said Justice Ashok Bhushan, who read out the judgment.

The three men - Pawan Gupta, Vinay Sharma and Mukesh Singh - had asked the Supreme Court to consider less severe punishment. A fourth man, Akshay Thakur, did not petition the court to review his death sentence.

Following the 2012 case, India launched fast-track courts and a tougher rape law in April this year that included the death penalty for rape of girls below 12.

Neverthless, more rape cases have come to light since then, and it remains hard for victims to receive timely justice.

The conviction rate of people arrested for rape was 25 percent in 2016, while the backlog of rape cases pending trial stood at more than 133,000 by the end of that year, up from about 100,000 in 2012.