The sexual assault took place Tuesday night at Bannu Nagaria village here when the sisters had gone to answer nature's call, a police official said.
The two girls, one 15 and the other 17, were beaten up by the attackers. One of them was seriously injured.
The incident took place around 9 pm when three unidentified persons dragged them to a far off field and allegedly gangraped them, said Om Prakash Gautam, in-charge of the Madnapur police station.
When the girls resisted, they were beaten.
Police filed a case against three unidentified men based on the father's complaint, circle officer Tilhar Arun Chandra said. A manhunt has been launched to nab the culprits, he added.
A medical examination of both girls will be done.
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