India's top court on Friday upheld the acquittal of a Bollywood director accused of raping an American research scholar in a case that has sparked intense debate about consent in a country with high levels of sexual violence.

Mahmood Farooqui was initially found guilty of rape in 2016, but the Delhi high court last year overturned the conviction on appeal, ruling the incident had been consensual.

One of the judges hearing that appeal said Farooqui may not have been aware that the woman had not consented to sex, a comment that attracted fierce criticism from rights activists.

"In cases where the parties are known to each other, it could be really difficult to decipher whether a feeble 'no' – little or no resistance – actually amounts to denial of consent," said Justice Ashutosh Kumar.

The alleged victim sought to appeal, but the Supreme Court on Friday dismissed her plea, saying the acquittal had been sound.

"The high court judgement is well written. It does not require our interference," said S.A. Bobde, one of the judges hearing the case.

Activist Kavita Krishnan said Friday's ruling was a "betrayal of women's rights" and of the new, tougher laws on sexual violence introduced after the fatal gang rape of a Delhi student in 2012.

"If you made a drink for a man, our Supreme Court thinks your No can then be read as Yes," tweeted Krishnan, secretary of the All India Progressive Women's Association.

"SC refusal to even admit the plea against atrocious 'Feeble No' verdict is a betrayal of women's rights and of the 2013 rape law."

The case dates back to 2015 when the scholar had travelled to India to seek Farooqui's assistance with her research.

She travelled home to the US shortly afterwards but returned to India to report the matter to police.

India has a grim record of sexual crimes against women, with nearly 39,000 rape cases reported in 2016, according to government data.

The 2012 Delhi gang rape sparked mass protests and led to an overhaul of rape laws that increased penalties for offenders and accelerated trials through courts.

But activists say much more needs to be done.