The incident took place in Kalandarpur village in Motipur Ranja of Katarniaghat wildlife division on Sunday, they said.
The girl, Anshika, was dragged away by the leopard while she was playing with her uncle outside her house on Sunday night, Divisional Forest Officer (DFO) Akashdeep Badhawan said.
Her severed head was found around 2 am in the forest, he said, adding that the rest of the body had still not been found.
On Friday, the leopard had killed a seven-year-old child about 1.5 km from the spot where Sunday's incident took place. Efforts were on to catch the leopard with the help of a drone camera, Badhawan said.
A tranquillizing team has also been alerted, he said.
The forest department team is asking villagers not to come out of their houses in the night and keep children inside homes, the DFO said.
In the past two months, over half a dozen leopard attacks have been reported in this area and two children have died in such incidents, Badhawan said.