Five Indian men held in connection with the alleged rape and murder of two teenage girls found hanging from a tree in May will be released without charge this week.

The girls, aged 14 and 15, were allegedly attacked when they went to relieve themselves in the fields near their home in Uttar Pradesh, in northern India.

Three of the men are related, and admitted to the rape and murder shortly after the girls were found.

The two other men held in custody are policemen who had allegedly failed to investigate the case when the girls were found because of their low caste.

Having been held in prison for 90 days, all five men will be released without charge on the grounds of insufficient evidence for prosecution.

They are expected to be released on Friday, having been held for the maximum length of time possible without charge.

However, the Central Bureau of Investigation, who took responsibility for the investigation after police failings, said that they had not ruled out the possibility of future charges.

"Based on our investigations so far, we are not going to file the charge-sheet at this stage. However, no one has been given a clean chit as yet," CBI spokeswoman Kanchan Prasad told AFP.

There are conflicting reports about the alleged rape. A post-mortem by a team of local doctors concluded that the girls had been sexually assaulted. But although the bodies were not able to be examined by the CBI last month – as their graves have been flooded – an examination of the girls' clothing found no proof of sexual assault.

Questions have also been raised about the reliability of a key witness.

There are concerns that their murder was an honour killing, although these reports have been denied by the family.

A fortnight after the two girls were found, another woman was found hanging from a tree in Uttar Pradesh.

Another incident occurred in Uttar Pradesh this month, when a 40-year-old Dalit woman was gang-raped as she was walking home. Her husband and son, who were walking with her, were knocked unconscious before she was raped.

Sexual violence in India has attracted greater international scrutiny since a female student was raped by a gang on a bus in Delhi in 2012.

Indian finance minister Arun Jaitley came under attack last week for describing the Delhi rape as a "small incident" which has cost the country billions of dollars in lost tourism.

In the attack two years ago, a 23-year-old physiotherapy student was repeatedly raped by six men on a moving bus, thrown naked from the vehicle, and later died from her injuries.

Jaitley said at a gathering of state tourism ministers on Thursday: "One small incident of rape in Delhi advertised [the] world over is enough to cost us billions of dollars in terms of lower tourism."

The number of rapes recorded by the Indian National Crime Bureau has increased by 7.1 per cent since 2010. It is estimated that a woman is raped in India every 20 minutes.

The penalty for rape in India has increased since the 2012 incident, and is now punishable by death.