The BSF personnel who were deployed to guard the Maphou dam escaped unharmed in the remote-controlled blast that took place around 9 a.m.
The girl, Bembem, was rushed to a private hospital. She was out of danger now, sources said.
Police and paramilitary officials rushed to the site, sources said.
The attackers escaped through the Maphou mountain in Ukhrul district. No combing operation was launched since the police feared the militants might have escaped to their camps.
Deputy Chief Minister Gaikhangam Gangmei instructed all the police stations as well as camps of the paramilitary forces and central forces to be on alert.
Director General of Police L.M. Khoute has ordered all police stations and Manipur Rifles camps to be on duty round the clock. Leaves of all security personnel have been cancelled.
Wednesday's attack comes after a series of bomb blasts in Manipur ahead of the Independence Day celebrations. The insurgent groups try to sabotage the celebrations every year.
Last week one security personnel and a two-year-old girl were injured when a bomb was detonated in the heart of Imphal city. One bomb was also recovered the following day.
Two powerful bombs were also exploded at the border town of Moreh on Monday. There was no casualty, officials said.
(With inputs from IANS)