Five men have been arrested over the brutal gang-rape and murder of two girls found hanging from a mango tree in northern India, police said Saturday.

The discovery Wednesday of the bodies of the cousins, aged 14 and 12, in impoverished Katrashadatganj village in Budaun district of Uttar Pradesh, marked the latest sexual violence case to stir national outrage.

"Rape of both girls has been confirmed. Cause of death was asphyxia," Budaun district police superintendent Atul Saxena told AFP.

Preliminary murder and gang-rape charges were filed against three men while two policemen are accused of being criminal accessories, Saxena said.

"These men (accused of murder) first raped the victims and then hung them as now confirmed by post-mortem reports," Saxena said.

"The girls were still alive when they were hung," Saxena said by phone from the district, 300 kilometres (180 miles) from Uttar Pradesh capital Lucknow.

He said all those named in a police complaint by the families had been arrested and police were probing whether more were involved.

The farm labouring family of the two cousins told AFP police could have "saved" the girls but said they refused to help when they found they were from a low caste.

"She was my everything, my world – and now my world has come to an end," the grief-stricken father of one victim said. The alleged attackers were from a higher caste than the girls.

There is a long history of women and girls from India's lower castes – especially those who are Dalits, previously known as "untouchables" – being sexually abused by people from higher castes.

"These policemen didn't act for hours when they could have saved two young lives. Why is caste everything?" said the father, who cannot be named for legal reasons.

The victims were sexually assaulted multiple times, police said.

Sex attacks common

India toughened sex assault laws following the fatal December 2012 gang-rape of a student on a bus in New Delhi which triggered nationwide protests, but the move has done little to stem the tide of sex attacks.

According to Indian government statistics, a rape happens every 22 minutes but activists say the figure is conservative as many rapes go unreported in the conservative nation of 1.2 billion people.

The father said his daughter and her cousin were attacked when they went to the fields to relieve themselves as there was no toilet in their house.

The father told AFP his brother heard screams from the field where the girls had gone and got into a scuffle with five men, but fled when they threatened to shoot him.

The family reported the crime to police who told relatives the girls were with an upper caste village man and would be back "in a couple of hours", the father said.

Then they got a call from a woman saying their daughters' bodies were hanging from a tree.

"This was nothing but plain murder and (caste) conspiracy," he said. Indian law forbids the naming of the victims or their families.

"The cops were totally hand-in-glove in getting our daughters killed," said the father of the other victim.

Rights activists said the crimes highlighted Uttar Pradesh authorities were "not serious" about tackling sexual crime.

The chief of Uttar Pradesh's ruling party sparked uproar last month when he opposed recent legislation making gang-rapists subject to capital punishment.

"Boys will be boys," Mulayam Singh Yadav said.

Amnesty International has said a lack of toilets across India forces women to answer the call of nature outside, "making them more vulnerable to violence".

Rahul Gandhi, who led India's Congress party to a crushing defeat by the Hindu nationalist Bharatiya Janata Party in national elections earlier in the month, visited the girls' families Saturday.

Gandhi said the victims' families told him they were only interested in "justice" for their daughters, not financial compensation.

It came as reports of two other rapes in Uttar Pradesh surfaced. In one case, police arrested three men over the gang-rape of a young woman that was later circulated in a video. The other case involved the alleged rape of a deaf-mute teenager by a neighbour.